Ph.D. THESIS - UFRN

FEDERAL UNIVERSITY OF RIO GRANDE DO NORTE

TECHNOLOGY CENTER

GRADUATE PROGRAM IN MATERIALS SCIENCE AND ENGINEERING

Ph.D. THESIS

INFLUENCE OF THE MICROSTRUCTURE ON THE POLISHING PROCESS OF

PORCELAIN STONEWARE TILES CONTAINING INDUSTRIAL WASTES

(Influência da microestrutura no processo de polimento de porcelanatos produzidos com

resíduos industriais)

JOSÉ ELSON SOARES FILHO

NATAL – RN

May 2018

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Thesis submitted to the Graduate Program in Materials

Science and Engineering of the Federal University of Rio

Grande do Norte in partial fulfilment of the requirements for

the degree of Ph.D. in Materials Science and Engineering.

Advisor: Prof. Dr. Eng. Rubens Maribondo do Nascimento

Co-advisor: Prof. Dr. Eng. Fábio José Pinheiro Sousa

Co-advisor in Germany: Prof. Dr. Ing. Jan Christian Aurich

NATAL – RN

April 2018

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FEDERAL UNIVERSITY OF RIO GRANDE DO NORTE TECHNOLOGY CENTER






Ph.D. thesis submitted to the evaluation committee of the Graduate Program in Materials

Science and Engineering of the Federal University of Rio Grande do Norte in partial fulfillment

of the requirements for the degree of Ph.D. in Materials Science and Engineering.

IV

V

To my sweet princess,

Bianca Falcão.

Daddy loves you!

VI

“What we know is a drop,

what we don't know is an ocean.”

Isaac Newton.

https://www.goodreads.com/author/show/135106.Isaac_Newton

VII

ACKNOWLEDGEMENTS

The present thesis has been carried out as a collaborative research between the Federal

University of Rio Grande do Norte - UFRN, in Natal-RN - Brazil, and the Technische

Universität Kaiserslautern TU-KL, in Kaiserslautern – Germany, within the UFRN / TU-KL

binational cooperation agreement. I really would like to express my gratitude to both

universities for providing me material and laboratory structure as well as important human

resources that I needed to conduct the work.

I am deeply grateful for the funding of the Coordination of Improvement of Higher

Education Personnel - CAPES and of the Deutsche Forschungsgemeinschaft - DFG (German

Research Support Society) within the framework of the Postgraduate Program in Science and

Engineering of Materials and the Collaborative Research Center 926 (SBF): Microscale

Morphology of Component Surfaces (MICOS), respectively.

Honestly, I have no words to thank all the people whose paths crossed mine during this

journey, but I really would like to express my sincere thanks.

In the scope of my doctoral project:

To my research advisors in Brazil, Prof. Dr. Fábio Pinheiro and Prof. Dr. Rubens

Maribondo for having dedicated their precious time in guidance and constant support in the

scientific and administrative aspect, helping me to conduct and complete the doctoral project. I

also would like to thank Prof. Dr. Carlos Paskocimas for his generous advice and scientific

discussions about the topic of my research and others topics. By working in their footmarks, I

was able to learn many lessons about ceramic materials and science in general.

To the Institute for Manufacturing Technology and Production System (FBK -

Fertigungstechnik und betriebsorganisation Kaiserslautern), especially, to prof. Jan C. Aurich,

who kindly welcomed me into the institute. The FBK team, represented by the team leader

Benjamin Kirsch and some members as Dinesh Setti, Martin Bohley, Peter Arrabiyeh, Marco

Zimmermann, Lukas Heberger, Stephan Basten, Stephan Gutwein and Jörg Hartig. To the

Administration and support staff, represented by Rosemarie Schleret, Marion Teubner and

Michael Lutzke. A special acknowledgment to my friends Patrick Mayer and Luciana Ninni

Schäfer. Dankeschön!

In Natal – Brazil, I thank the student workers Vanessa Almeida, Erick Ferreira and

Thomas Monteiro for the experimental assistance during the ceramic processing carried out at

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the laboratory of ceramic materials, as well as all support from all laboratories that I have

been using the structure.

To the raw materials suppliers: Elizabeth Cerâmica for supplying the abrasives used in

some polishing tests. Armil – Mineração do Nordeste for supplying the ceramic raw materials

and some wastes. Mr Gabriel Souza for supplying the kaolin and its waste. Mr Arivam for the

ball clay used in the project. Mining Tomaz Salustino for supplying the waste from the Sheelite

extraction.

In the familiar scope:

Firstly, to my base, my fortress, my mother, Fatima Falcão. Since I was 9 years old, she

took on the heavy responsibility of being father and mother together. I know it was not easy to

take care of five children, but I am sure that today you are getting great results, a result of your

discipline and dedication with us. Thank you! I love you!

To my late father, Jose Elson, whose name I carry with great pride. Unfortunately, we

had little time together, however, enough for me to grasp valuable lessons of citizenship. I miss

you so much!

To my little treasure, Bianca Falcão, who four years ago fills my heart with happiness.

Daddy loves you!

My wife Heloisa Almeida, for the love and affection dedicated to me, for the

understanding and exclusive dedication to our daughter in the moments in which I was absent.

I love you!

My sisters Adriana, Elciane and Aline for continued encouragement. To my brother

Elton for his fellowship. I have the conviction that I can always count on you and know that

reciprocal is also true!

Finally, to all my friends of the Postgraduate Program in Materials Science and

Engineering of UFRN, with whom I was able to participate in several academic-scientific

discussions.

IX

ABSTRACT

Nowadays the concern with the conservation of the natural sources of raw materials is in

evidence, generating a notorious environmental awareness worldwide. For a long time, the

society discusses the importance of environmentally friendly products. In this context, the

ceramic tiles sector is perfectly capable of absorbing a range of industrial wastes in ceramic

tiles compositions. Since the last two decades, research has been carried out aiming at obtaining

eco-friendly ceramic tiles, including porcelain stoneware tiles, from industrial wastes as

alternative raw materials. However, most of these investigations on the technical feasibility of

the insertion of industrial waste were focused on technological and mechanical performance.

The behavior regarding the glossiness gain of eco-friendly ceramic tiles remains unknown both

in the field of scientific research and in the industrial scope. Thus, the present work aims to

evaluate the influence of the microstructural features, caused by the insertion of industrial

wastes, on the final surface quality, considering the polishing process. For this purpose, eco-

friendly porcelain stoneware tiles compositions were developed from different industrial wastes

from the state of Rio Grande do Norte, Brazil. The surface characteristics prior to polishing

were characterized and 3D mapped, in order to obtain an estimate of the minimum thickness to

be removed during the polishing process. Values of material removal of 1.08 up to 2.37% of

the initial thicknesses were found to mitigate the roughness and waviness effects of the samples.

During the polishing process, the kinematic parameters were kept constant, as well as the

polishing time intentionally saturated, aiming to ensure that the microstructural difference is

the only variable to be related to the final glossiness reached. In total, six eco-friendly

compositions were polished, three of them with different proportions (5, 10 and 15 wt.%) of

the waste from the breakage of bricks and roof tiles and another three from different proportions

(10, 15 and 20 wt.%) of the waste from the beneficiation of primary kaolin ore. The surface

quality of the compositions was characterized by determination of glossiness and roughness

curves as well as the morphology of the samples. The patterns were compared to a waste-free

composition. The experimental results have shown that it was possible to raise the glossiness

level of 59 gloss units, for the composition waste-free, to levels between 66 and 72 gloss units

reached for the eco-friendly porcelain tiles. These values are close to those generally found in

commercial porcelain tiles. Such results are able to validate the potential of using industrial

wastes as alternative raw materials for the production of polished ecological porcelain tiles and

enable the direct transfer of knowledge to the productive sector, contributing to the

diversification of the Brazilian industrial matrix in the direction of technologically more

advanced products, of greater added value and ecologically correct.

Keywords: polishing process, microstructure, wastes, incorporation, eco-friendly porcelain

stoneware tiles

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RESUMO

A preocupação com a conservação das fontes naturais de matérias-primas está em evidência,

gerando uma notória consciência ambiental no mundo inteiro. A muito tempo se discute a

importância de produtos ecologicamente corretos. Neste contexto, o setor de revestimentos

cerâmicos é perfeitamente capaz de absorver uma gama de resíduos industriais em suas

composições. Há aproximadamente duas últimas décadas, pesquisas têm sido realizadas

visando a obtenção de revestimentos cerâmicos ecologicamente corretos com a tipologia de

porcelanato a partir da incorporação de resíduos industriais como matérias-primas alternativas.

No entanto, a maioria dessas investigações sobre a viabilidade técnica da inserção de resíduos

industriais ficaram focadas no desempenho tecnológico e mecânico. O comportamento em

relação ao ganho de brilho de revestimentos cerâmicos ecológicos permanece desconhecido

tanto no campo da pesquisa científica quanto no âmbito industrial. Assim, o presente trabalho

tem como objetivo principal avaliar a influência da diferença microestrutural, causada pela

inserção de resíduos industriais, na qualidade da superfície final, considerando o processo de

polimento. Para tanto, foram desenvolvidas composições distintas de porcelanato a partir de

diferentes resíduos industriais do estado do Rio Grande do Norte, Brasil. As características das

superfícies antes do processo de polimento foram mensuradas e mapeadas em 3D, afim de se

obter uma estimativa da espessura mínima a ser removida durante o processo de polimento.

Foram mensurados valores de 1,08 à 2,37% das espessuras iniciais a serem removidas para

atenuar os efeitos de rugosidade e ondulação das amostras. Durante o polimento, os parâmetros

cinemáticos foram mantidos constantes, assim como o tempo de polimento foi

intencionalmente saturado, a fim de garantir que a diferença microestrutural seja a única

variável a ser relacionada ao brilho final atingido. No total, seis composições ecologicamente

corretas foram polidas, três delas com diferentes proporções (5, 10 e 15% em massa) de resíduos

da quebra de tijolos e telhas e outras três de diferentes proporções (10, 15 e 20% em massa) do

resíduo do beneficiamento do minério de caulim primário. A qualidade superficial das

composições foi caracterizada pela determinação das curvas de brilho e rugosidade, bem como

pela morfologia das amostras, via mapeamento 3D. Os padrões foram comparados a uma

composição livre de resíduos. Os resultados experimentais mostraram que foi possível elevar o

nível de brilho de 59 unidades de brilho para a composição isenta de resíduos a níveis variando

de 66 até 72 unidade de brilho, para os porcelanatos ecológicos. Tais valores são próximos aos

geralmente encontrados em porcelanatos comerciais, sendo assim, os resultados validam o

potencial de utilização de resíduos industriais como matérias-primas alternativas para a

produção de porcelanatos ecológicos polidos e possibilitam a transferência direta de

conhecimento para o setor produtivo, contribuindo para a diversificação da matriz industrial

brasileira na direção de produtos tecnologicamente mais avançados, de maior valor agregado e

ecologicamente corretos.

Palavras-chave: processo de polimento, microestrutura, resíduos, incorporação, porcelanatos

ecológicos.

XI

LIST OF FIGURES

Figure 1: Overview of Brazilian ceramic tiles production throughout the years 2010 to 2016.

Author (adapted from ANFACER). ...................................................................................... 21

Figure 2: HD porcelain stoneware tiles simulating marble (a) and wood (b). Available at

magazine "Link Elizabeth - launch 2017", Elizabeth Cerâmica. ............................................ 22

Figure 3: Flowchart of porcelain stoneware tile production in the polished version. .............. 23

Figure 4: A public seat at Luisenpark in Mannheim, Germany. Detail of the natural and

polished surfaces of a granitic rock. Author. ......................................................................... 26

Figure 5: Waviness and roughness exemplification. Author. ................................................. 26

Figure 6: Cement abrasive (new) and diamond abrasive (used). Author. Courtesy of Elizabeth

Cerâmica S.A. ...................................................................................................................... 28

Figure 7: Representation of the abrasives mounted to the polishing head. Detail of the swing

motion of the abrasive block. Author. ................................................................................... 28

Figure 8: Cracks system by the polishing of ceramic floor tiles. Author. ............................... 29

Figure 9: Wear mechanism in porcelain stoneware tiles, as proposed by Sousa, 2014. Author.

............................................................................................................................................. 30

Figure 10: Industrial polishing train. Courtesy of Elizabeh Ceramic, S.A. ............................. 31

Figure 11: Typical asymptotic curves of gloss and roughness, as defined by Hutchings

(Hutchings et al., 2005b)....................................................................................................... 33

Figure 12: Radial motion (a) and transverse motion (b) with respective generated glossiness

patterns. Adapted from Cantavela, 2004 and Sousa, 2007b. .................................................. 34

Figure 13: X–ray diffraction patterns of the wastes. .............................................................. 51

Figure 14: Thermogravimetric and differential thermal analyses of CK. ............................... 52

Figure 15: Thermogravimetric and differential thermal analyses of FK. ................................ 53

Figure 16: Thermogravimetric and differential thermal analyses of GP. ................................ 53

Figure 17: Thermogravimetric and differential thermal analyses of TB. ............................... 54

Figure 18: Thermogravimetric and differential thermal analyses of CW................................ 55

Figure 19: Water absorption (a) and flexural strength of compositions. ................................. 57

Figure 20: Microstructures (as seen on SEM) of porcelain stoneware tiles obtained from: (a)

composition 2, (b) composition 4. Details showing mullite needles (1), quartz grains (2),

porous (*) and micro-cracks. ................................................................................................ 58

Figure 21: Microstructures, as seen on SEM, of porcelain stoneware tiles obtained for

composition 6. Details showing mullite needles (1), quartz grains (2) e porous (*). .............. 59

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Figure 22: Microstructures, as seen on SEM, of porcelain stoneware tiles obtained for

composition 8. Details showing mullite needles (1), quartz grains (2) e porous (*). .............. 59

Figure 23: Microstructure (as seen on SEM) of porcelain stoneware tiles obtained for

composition 9. Details showing mullite needles (1), quartz grains (2), “ceramic foam” (3), and

porous (*). ............................................................................................................................ 60

Figure 24: Laser triangulation in porcelain tiles with the detailed surface profile. ................. 66

Figure 25: : Illustration of waviness versus roughness profiles in tiles surfaces. .................... 67

Figure 26: Evaluated surfaces with the investigated area marked in red. ............................... 70

Figure 27: Micrometric characterization of the surface for composition 1. ............................ 71






Figure 33: a) Cement fickert (new) and b) diamond fickert (used). Author. ........................... 80

Figure 34: Representation of the fickerts mounted to the polishing head. Detail of the swing

motion performed by the fickert. Author. .............................................................................. 81

Figure 35: (a) Polishing machine adopted. (b) Detail of the motion sources (schematic plan

view)(c) Detail of the loading system (schematic side view). ............................................... 83

Figure 36: Glossiness and roughness measuring points, including the direction of the

measurements. ...................................................................................................................... 84

Figure 37: Microstructure, as seen by SEM, of the composition free of waste. ...................... 85

Figure 38: Microstructure (as seen by SEM) of the compositions C5 – (a), C10 – (b) and C15

– (c) with details of EDX analysis for three different positions each one. .............................. 86

Figure 39: Gloss (a) and roughness (b) development through the abrasive sequence for the

stardard composition............................................................................................................. 87

Figure 40: Surface gloss development through the abrasive sequence for the compositions

with waste incorporated. ....................................................................................................... 88

Figure 41: Surface roughness development through the abrasive sequence for the

compositions with waste incorporated. ................................................................................. 88

Figure 42: Surface evolution of composition C5. .................................................................. 89

Figure 43: Surface evolution of composition C10. ................................................................ 90

Figure 44: Surface evolution of composition C15. ................................................................ 90

file:///C:/Users/Elson/Desktop/Thesis_Elsonversao%20Final.docx%23_Toc518941560


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Figure 45: Interactions between the abrasives particles and the porcelain stoneware tiles

surface. Author. .................................................................................................................... 95

Figure 46: Triaxial system (wt.%) for ceramic products with the mixtures within the porcelain

stoneware tile region, with detail of the interested region. ..................................................... 97

Figure 47: (a)Polishing machine adopted. (b) Detail of the system mounted. (c) Detail of the

adaptation (schematic side view). ......................................................................................... 98


measurements. ...................................................................................................................... 99

Figure 49: SEM Micrographs of sintered samples from the compositions STD, 10%, 15% and

20%. (15,000x magnification)............................................................................................. 100

Figure 50: Glossiness and roughness curves for the STD composition. ............................... 101

Figure 51: Glossiness curves for the eco-friendly compositions. ......................................... 103

Figure 52: Roughness curves for the eco-friendly compositions. ......................................... 104

Figure 53: Topography of the composition C10: (a) Fired surface (natural condition) and (b)

polished surface. ................................................................................................................. 105

Figure 54: Topography of the composition C15: (a) unpolished surface and (b) polished

surface. ............................................................................................................................... 105


surface. ............................................................................................................................... 106

Figure 56: Optical image of the eco-friendly compositions. ................................................ 107


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LIST OF TABLE

Table 1: Classification of ceramics floor tiles based on water absorption and flexural strength,

according to the standards. .................................................................................................... 17

Table 2: Lapidation and polishing features. Author. .............................................................. 25

Table 3: Relation between grit number and the average size of the abrasive particles used in

the polishing operation. Author. ............................................................................................ 31

Table 4: Compositions developed. ........................................................................................ 48

Table 5: Chemical analysis of the wastes used, as determined by XRF. ................................ 49

Table 6: Chemical and phase mineralogical characteristics of the raw materials used. ........... 68

Table 7: Compositions of inspected porcelain tiles................................................................ 69

Table 8: Compositions adopted and their resulting technological properties. ......................... 82

Table 9: Kinematics parameters adopted. .............................................................................. 83

Table 10: Compositions adopted and their resulting technological properties. ....................... 96

Table 11:Abrasive sequence adopted. ................................................................................... 98

Table 12: Kinematics parameters adopted. ............................................................................ 98

XV

LIST OF ABREVIATIONS

ABNT Associação Brasileira de Normas Técnicas (Brazilian Association of Technical

Standards)

ANFACER Associação Nacional dos Fabricantes de Cerâmica para Revestimentos, Louças

Sanitárias e Congêneres (National Association of Manufacturers of Ceramic Tiles and Sanitary

Articles)

ASTM American Society for Testing and Materials

CCD Charged Coupled Device

CK Coarse Kaolin

CW Waste from processing of calcium tungstate mineral (CaWO4)

DTA Differential Thermal Analisys

EDX Energy-dispersive X-ray spectroscopy

EN European Standards

FBK Fertigungstechnik und Betriebsorganisation (Institute for Manufacturing

Technology and Production Systems)

FK Fine Kaolin

FS Flexural Strength

GP Granitic Powder

ISO International Organization for Standardization

ICDD International Centre for Diffraction Data

JCPDS Joint Committee on Powder Diffraction Standards

LS Linear shrinkage

LCD Liquid Crystal Display

NBR Norma Brasileira (Brazilian Standard)

SEM Scanning Electron Microscopy

STD Standard

TB Tiles and Bricks

TG Thermogravimetric

WA Water absorption

XRD X-ray Diffraction

XRF X-ray Fluorescence

XVI

SUMMARY

1 INTRODUCTION .................................................................................................................. 17

1.1 MOTIVATION ................................................................................................................. 17

1.2 GOALS ............................................................................................................................. 19

2 BACKGROUND ..................................................................................................................... 20

2.1 BRAZILIAN MARKET FEATURES ................................................................................ 20

2.2 PORCELAIN STONEWARE TILE MANUFACTURING PROCESS ............................... 23

2.3 POLISHING PROCESS .................................................................................................... 25

2.3.1 ABRASIVE ENVIRONMENT .................................................................................. 27

2.3.2 ABRASIVE WEAR AND MATERIAL REMOVAL ON CERAMIC MATERIALS . 29

2.3.3 INDUSTRIAL POLISHING OPERATION ............................................................... 30

2.4 SCIENTIFIC BREAKTHROUGHS IN PORCELAIN POLISHING .................................. 32

2.5 REUSE OF WASTES IN THE CERAMIC MATRIX ........................................................ 35

2.6 REFERENCES .................................................................................................................. 40

3 RESULTS AND DISCUSSION .............................................................................................. 45

3.1 HIGH-PERFORMANCE “GREEN” CERAMIC TILES FORMULATIONS WITH

INDUSTRIAL INORGANIC WASTES ........................................................................................ 46

3.2 ESTIMATION OF THE MINIMUM MATERIAL REMOVAL THICKNESS DURING THE

POLISHING PROCESS OF CERAMIC TILES BY LASER TRIANGULATION ........................ 64

3.3 POLISHING PERFORMANCE OF GREEN CERAMIC TILES MADE WITH WASTES

FROM BRICKS AND ROOF TILES ............................................................................................ 79

3.4 EVALUATION OF SURFACE QUALITY AFTER POLISHING OF ECO-FRIENDLY

PORCELAIN STONEWARE TILES MADE WITH KAOLIN ORE WASTE .............................. 94

4 FINAL CONSIDERATIONS ................................................................................................ 110

5 CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK ....................................... 112

5.1 CONCLUSIONS ............................................................................................................. 112

5.2 SUGGESTIONS FOR IMPROVEMENTS AND FUTURE INVESTIGATIONS............. 114

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1 INTRODUCTION

1.1 MOTIVATION

Since the last decade, Brazil has become one of the giant ceramic products producers,

especially ceramic floor tiles, taking the second position of largest producer and consumer in

the world (ANFACER, 2017),(ABCERAM, 2017). Porcelain stoneware tile is the most

advanced product among traditional ceramic floor tiles. In addition, it is undergoing progressive

evolution in the research field into the development and optimization of production processes

including finishing, due to the huge global demand (ANFACER, 2017). It is a high-performance

and quality ceramic tile, consequently having higher added value.

Such product presents high-performance as surface hardness, mechanical strength,

water absorption (≤ 0.5% for glazed version and ≤ 0,1% for polished version (ABNT, 1997))

and low surface porosity, and a microstructure that provides high resistance to chemical attack

and staining (Dondi et al., 2005)(Suvaci and Tamsu, 2010)(Cavalcante et al., 2004). Besides,

the aesthetic value of such tiles can be greatly enhanced when they undergo a polishing process

to achieve high glossiness (Hutchings et al., 2005a).

The most important parameters to classify the different types of ceramic floor tiles are

those that refer to the level of water absorption (WA) and mechanical resistance, represented

by the flexural strength (FS). Both parameters are provided in different standards as described

in Table 1.

Table 1: Classification of ceramics floor tiles based on water absorption and flexural strength,

according to the standards.

Water Flexural Typology

Absorption - WA (%) Strength - FS (MPa) ABNT 13817 ISO 13006 ASTM C973-88

WA ≤ 0,5% FS ≥ 35 Porcelain stoneware BIa Very vitrified

0,5% < WA ≤ 3% 30 ≤ FS < 35 Stoneware BIb Vitrified

3% < WA ≤ 6% 22 ≤ FS < 30 Semi-stoneware BIIa Semi-vitrified

6% < WA ≤ 10% 15 ≤ FS < 18 Semi-porous BIIb Non Vitrified

WA > 10% FS ≤ 15 Porous BIII

Nowadays, one of the biggest concerns of humanity is the conservation of nature. For

a long time is discussed new routes for the sustainable development and management of

industrial waste. The exploitation of ceramic raw materials is accountable for the production of

large quantities of wastes. In Brazil, several mounds of ceramic wastes can be found disposed

18

along the main roads and in some cases without considering their environmental impacts. The

increasing industrial demand (Andreola et al., 2016) for such raw materials leads to more

intense exploitation activities. As a consequence, the generation of tailings from mining and

other industrial activities increases, and so does the interest in reusing them (Menezes et al.,

2008)(Kazmi et al., 2016)(Teo et al., 2014)(Novais et al., 2015). On the other hand, the ceramic

floor tile industry is considered flexible and versatile under the ecological point of view. It is a

waste generator, nevertheless, it is capable of reusing a large variety of residues as additives or

alternative raw materials. The sector plays an important role in the reuse of wastes that may

have deep similarities in chemical and mineralogical compositions with conventional raw

materials (Andreola et al., 2016),(Menezes et al., 2009). Several studies have reported the

incorporation of mining and industrial wastes into the ceramic matrix, in the formulations of

various products such as cement (Elçi, 2016), sealing blocks, roof tiles (Dondi et al., 2009), and

porcelain stoneware tiles (Leite and Almeida, 2015)(Soares Filho et al., 2014)(Dos Santos et

al., 2014)(Gouvêa et al., 2015)(Penteado et al., 2016).

The Brazilian state of Rio Grande do Norte stands out among the other states of the

northeast region due to the large deposits of feldspathic rocks, kaolinitic clays, and kaolin,

which are the main raw materials for the porcelain stoneware tiles production. Even with good

sources of natural raw materials, the main ceramic products manufactured in the Rio Grande do

Norte are lower added value, such as semi-vitrified and non-vitrified bricks, blocks, and tiles.

In view of the potential of the Brazilian Northeast region, especially the state of Rio

Grande do Norte, several researches were developed aiming to obtain porcelain stoneware tiles

from the state's own raw materials, as well as the use of industrial-scale wastes (Acchar et al.,

2013) (Soares Filho et al., 2014). Meanwhile, technical feasibility assessments were focused on

technological and mechanical properties. The behavior regarding the glossiness of the obtained

products and the influence of the microstructure in the polishing process is still unknown.

Among the many versions of porcelain stoneware tiles that the companies develop and

launch annually in the market, the polished version is still the most appreciated by the final

consumers, being the surface gloss considered the criterion of quality of major importance. In

this way, the polishing stage is extremely important, since it is the stage responsible for the final

appearance of the porcelain tile and has a direct impact on the commercialization and consumer

satisfaction. Thus, it is fundamental to carry out research on the subject in order to understand

the mechanisms involved for optimization of the process and the influence that the variation of

microstructure in this stage.

19

Usually, the industrial polishing process is composed of a sequence of 14 to 18 different

abrasive grain sizes, generally between 36 and 2500 mesh. Considering the entire porcelain

production chain, the polishing process accounts for between 30-40% of all production costs

(Hutchings et al., 2005 and 2006). Due to the high consumption of abrasive tools, water, and

energy consumption are also high. Usually, 10% of the original thickness of the porcelain

stoneware tile is removed during this step. (Wiggers et al., 2007)(Sánchez et al., 2002).

The Brazilian ceramic floor tiles industry, following the world model, is moving towards

the development of technologically advanced, but ecologically friendly products. In this way,

the development of “green” formulations, ceramic processing and surface finishing via

polishing make up the central theme of this thesis. The knowledge to be gained on this subject

is of great use to the industries of this sector, considering that the polishing process is the most

expensive step in the production process, accounting for up to 40% of total costs.

1.2 GOALS

The main goal of this study is to conceive and polish porcelain stoneware tiles incorporating

industrial wastes from the state of Rio Grande do Norte. For this propose, the following specific

goals will be checked.

a. To conceive formulations with porcelain stoneware tile characteristics from raw

materials, and industrial wastes of Rio Grande do Norte state (coarse and fine wastes

from kaolin beneficiation process, waste from the extraction of granitic rocks for civil

construction, waste from processing and beneficiation of the calcium tungstate mineral

- CaWO4, and shards resulting from the breakage of red ceramic bodies).

b. To characterize formulations in technological and mechanical properties terms, in order

to attest the technical feasibility of obtaining porcelain stoneware tiles with the residues

used.

c. To obtain a surface analysis by 3D mapping at a pre-polishing step of the samples.

d. To obtain the curves for gloss and roughness performance throughout the abrasive

sequence.

e. To establish relationships between microstructure and glossiness.

20

2 BACKGROUND

2.1 BRAZILIAN MARKET FEATURES

The ceramic floor tile sector is more competitive each year, with a notable increase in

research and development, due to the international competition (mainly the Chinese market),

export and domestic demand. From the total of approximately US$ 300 million in investments

in Brazil by the ceramic industries in 2014 and 2015, about 70% were directed to the production

of porcelain stoneware tiles. This amount invested in that biennium stands for 18 new

production lines, according to the National Association of Manufacturers of Ceramic Tiles and

Sanitary Articles (ANFACER, 2017).

Currently, the Brazilian production of porcelain stoneware tiles accounts between 8%

and 10% of the volume of ceramic floor tiles produced, considered low when compared to Italy

production, for instance, where 70% of the total tiles produced are porcelain tiles (ANFACER,

2017). On the other hand, the Brazilian porcelain tiles have become an international reference

for quality and comply with the international standards. Furthermore, Brazil is the only country

that has a specific technical standard for porcelain tile. Created in 2007, the NBR 15.463

(ABNT, 2007) establishes specific technical parameters for the manufacture of these products

and protects the consumer from non-standard products. Practically 100% of the porcelain

stoneware tiles made in Brazil already meet the normative requirements, placing the Brazilian

product at a high-quality level.

Brazil is one of the main protagonists in the world market of ceramic tiles. After China,

Brazil is the second largest producer and consumer of ceramic floor tiles, followed by India,

Indonesia, Spain, and Iran. In exports, Brazil occupies the discrete 7th place with only 10.86%

of the total produced in 2016 aimed at the external market, which means that the country is able

to absorb almost all of its production. The main buyers of Brazilians tiles are countries from

South America, North America, and Caribe, an among around 180 countries (ANFACER,

2017).

Figure 1 shows the Brazilian production between the years 2010 and 2016, detailing

the exports in the year 2016, according to ANFACER.

21

Figure 1: Overview of Brazilian ceramic tiles production throughout the years 2010 to 2016.

Author (adapted from ANFACER).

In the current year, 2018, there are 103 ceramic tiles manufactures from 93 companies

operating in Brazil. Some regions are more favored and concentrate a considerable portion of

the producing units, due to factors such as the abundance of natural deposits, quality of raw

materials, availability of water and marketing. The south and southeast of Brazil hold

approximately 85% of the industries. The region of Criciúma, in Santa Catarina, is considered

as a ceramic cluster and agglomerates the largest Brazilian companies. On the other hand, the

state of São Paulo, the production is distributed in two poles: Mogi Guaçú and Santa Gertrudes

(ABCERAM, 2017).

The northeastern region, specifically the states of Bahia, Pernambuco, Paraiba, Rio

Grande do Norte, and Ceará, owns approximately 15% of the total factories and is in full

development and growth in manufacturing facilities, showing that it has potential and can

become a producing pole in the near future. This is due to favorable conditions such as high-

quality raw materials, viable energy, and a developing consumer market, as well as a privileged

geographic position to serve the central-west and north regions, as well as export to mainland

European countries, for example.

Following the recent innovation in the international industry, the Brazilian porcelain

tiles won a decorated version without any kind of ceramic glaze, but a digital decoration, called

HD (high definition). The pieces pass through a digital press where the decoration is printed on

the surface with resolutions from 180 to 1200 dpi. In this process, the porcelain tile is literally

printed like a sheet on an ink-jet printer. The polished HD porcelain stoneware tiles receive this

decoration before the firing step, and then they are polished. The design configuration of the

22

products mostly represent various natural elements such as marbles, stones and even wood.

Nevertheless, it is possible to "print" any image in a porcelain tile, which can be an image in a

single piece or even a mosaic of parts. Figure 2 shows different types of high gloss HD porcelain

stoneware tiles.

Figure 2: HD porcelain stoneware tiles simulating marble (a) and wood (b). Available at

magazine "Link Elizabeth - launch 2017", Elizabeth Cerâmica.

23

2.2 PORCELAIN STONEWARE TILE MANUFACTURING PROCESS

In order to achieve the properties required for such typology, the industrial production is

generally well automated with high-tech equipment, so that human interference is mostly

summarized in the design of formulations, process control, quality inspection of finished

product, storage, and shipment.

The industrial production is divided into main stages such as set out in the Fig 3.

Figure 3: Flowchart of porcelain stoneware tile production in the polished version.

The porcelain stoneware tiles formulations are composed typically of 20-50 wt.% kaolin

and kaolinitic clays, known as "ball clays", a similar proportion of sodium-potassium feldspars

or feldspar-like minerals and usually 10-15% of quartz added as a complement to quartz already

contained in raw materials (Sanchez et al., 2001). In addition, other raw materials and additives

are used, to a lesser amount, such as bentonite, alkaline earth carbonates, talc, sodium silicate,

24

and zirconia, in order to improve some properties of the product and assist the processing of the

composition.

The milling operation of the composition is carried out in a wet medium, in order to

obtain a suspension with around 60 wt.% solids content. The main control variables in the wet

milling process are density, viscosity and residue amount. A non-effective milling will

compromise the reactivity of the mixture during the firing step, which may cause particle

tearing during the polishing process, in addition, outcropping of pores closed to the surface

(Rosso et al., 2005). Moreover, the mechanical strength after firing is related to the size of the

microspores generated inside the part, which depends, among other factors, on the particle size

of the powder mass and the viscosity of the glassy phase during sintering.

Drying is performed by spraying. The process consists of dehumidifying by injection of

the nebulized suspension under high pressure and conveniently into a drying chamber in which

it is exposed to an air stream at a temperature ranging from 500 to 600 °C. The resulting

products are granules or agglomerates of smaller particles. The shape of those particles depends

greatly on the physical and chemical properties of the suspension, the characteristics of the

spray dryer and the operating conditions. According to Rosso (Rosso et al., 2005), the shape,

moisture and size of the granules are capable of affecting the fluidity of the powder and,

consequently, filling the mold in the press, which in turn can influence the mechanical

resistance to green of the parts, generate dimensional problems and cracks during heat

treatment.

Variations in particle size means variations in the hue of the finished product. The

resting time of the dried powder may not be less than 36 hours, in order to guarantee the

homogenization of the moisture (Moraes, 2007). The pressing process is a step by which the

ceramic body takes the final shape (square or rectangle). After compacting, the pieces acquire

mechanical strength, allowing them to withstand handling to the following treatments. For

porcelain stoneware tile, a specific compaction pressure in the range of 35 to 45 MPa is

recommended, which allows a density of 1.95 to 2.00 g/cm³.

The heat treatment involves two steps. The first one is drying, aiming to reduce the

humidity from the pressing step, usually between 6 to 8 wt.% to values below 0.5 wt.%. The

drying operation is performed in a continuous dryer by the circulation of hot air, which

generates the thermal gradients necessary for both mass transfer and heat transport (Barbosa et

al., 2008). This is an extremely important step for the prevention of defects such as cracks and

high porosity (Vicenzi and Bergmann, 2009).

25

The second step involves sintering. In this stage, all the effects of previous operations

are showed up. According to (Nicolau, 2012), the temperature distribution in the oven plays a

decisive role in obtaining the desired properties of the final product. In this stage of the process,

the thermal energy required to promote a series of chemical and physical reactions such as

thermal decomposition, allotropic transformations, liquid phase formation and sintering

(Cargnin et al., 2011) will be transferred. According to (Pinheiro and Holanda, 2013), some of

those reactions define the development of the final microstructure of the tile. After firing, a

porcelain tile is commonly composed of approximately 55-65 wt.% of a vitreous matrix, 20-25

wt.% quartz and 12-16 wt.% mullite (Baucia Jr et al., 2010).

2.3 POLISHING PROCESS

The polishing process in any piece, whether metallic or ceramic, is performed with a set

of tools made of segments of abrasive material, assembled in group. During this action, the

abrasive grits of the tool come in contact with the surface of the workpiece, rotating. In the

porcelain stoneware tile industry, this step is often known colloquially as "polishing", since the

finished surface has a high gloss. In general, the polishing process consists in generating gloss

on any surface, whether natural or manufactured, such as porcelain tiles.

The interest in porcelain polishing started in the 1980s. In 1987 the first radial polisher

was designed, with a capacity to polish approximately 20 m²/h of a 300x300 mm porcelain tile,

according to Vazques (Vazques, 2005). Currently, it is possible to polish pieces larger than

1000x1000 mm at a higher rate.

For a long time, since 1987, the knowledge of porcelain polishing was related to the

cutting process of precious or semiprecious stones (gems).However, through the years, much

has been learned about the phenomenological characteristics of porcelain tile polishing process,

so that it already possible to draw some fundamental differences (Sousa, 2007a), as shown in

Table 2.

Table 2: Lapidation and polishing features. Author.

Abrasive Characteristics

Diffuser Position Type Granulation Grit size (um)

Lapidation Liquid loose SiC, Al2O3, BN, diamond #600 - #1200 9.3 - 3.0

Polishing Soft solid Fixed SiC and diamond #36 - #1500 530.0 - <3

26

The surface finish of natural stones such as granite greatly contributed to what is now

known about porcelain polishing due to numerous similarities. For a long time, many industries

operated in the method of trial and error, based on the knowledge coming from this field. Figure

4 details the difference between polished and non-polished surfaces on a granitic rock.

Figure 4: A public seat at Luisenpark in Mannheim, Germany. Detail of the natural and

polished surfaces of a granitic rock. Author.

According to Sousa et al, (Sousa et al., 2014), the term polishing refers to a severe

elimination or reduction of roughness. However, in the ceramic tile industry, the term

"polishing" also refers to the elimination of another factor together, the waviness. The waviness

reduction is carried out on the surface leveling stage, subsequently. The effects of surface

roughness are reduced in the polishing machines, which entails a higher light reflectance,

consequently, the gloss development. Fig. 5 shows an illustration of the general surface of the

porcelain stoneware tile, with the two main parameters.

Figure 5: Waviness and roughness exemplification. Author.

27

The high-gloss polished porcelain stoneware tile is in increasing demand worldwide

(Sánchez et a 2002). In addition to the excellent mechanical performance, high resistance to

chemical etching, stain and wear resistance, the gloss is the aesthetic characteristic most

appreciated in this typology by the consumers. It is necessary to strict control in the stages of

ceramic processing, aiming at a homogeneity in the appearance aspect of the pieces, since no

material is deposited on the surface anymore, for example, ceramic glaze.

Considered one of the most important steps in the production, the polishing is often the

most costly step. According to previous researches (Matsunaga et al., 2014),(Hutchings et al.,

2005a), even after several studies to optimize this process and consequently to reduce costs,

this step is still responsible for 30-40% of the total cost of production, referring to water,

abrasive tool wear, and energy consumption.

The water consumption, used as a lubricant and coolant can reach 30 L / m² polished.

However, it is worth mentioning that currently almost 80% of the water used is treated by

filtering presses are reused in other production environments. Between 0.5 and 0.6 kg of

abrasive material/m² are required. As a consequence, considerable amounts of wastes are

generated. Currently 10% of the part is consumed in such operation, using from 0.5 to 0.6 kg

of abrasive/polished m² (Hutchings et al., 2005a). In view of the mentioned facts, the industrial

polishing process still can be considered as low efficient.

Once the heat treatment cycle of the pieces is completed, a levelling process is required

in order to eliminate the waviness generated during the previous firing step. After polishing,

the dimensional calibration (colloquially known as “squaring”) of the pieces is carried out by

grinding. This operation is also necessary for the same reason of levelling. For such steps from

2 to 4 grinding diamond cylinders are used. The diamond rolls cause severe damages to the

surface of the parts, which are partially removed by the action of one or more planetary grinding

machine, thus ensuring the flatness of the plates before the polishing step start (Wang et al.,

2003), (Esposito et al., 2005), (Wiggers et al., 2007).

For a better understanding of the industrial polishing process, it is necessary to know

the abrasive medium and the mechanisms of abrasive wear and removal of material in ceramic

materials.

2.3.1 ABRASIVE ENVIRONMENT

Currently, there are two main abrasive types used in the industrial polishing process.

Also known as fickerts, the abrasive blocks are composed of abrasive particles of silicon carbide

28

(SiC) or synthetic diamond, usually 10 wt.% (Hutchings et al., 2005b). Diamond particles are

dispersed in metallic matrix and SiC fickerts in cement matrix, usually magnesian (magnesium

oxychloride cement). Figure 6 shows the two types of abrasives.

Figure 6: Cement abrasive (new) and diamond abrasive (used). Author. Courtesy of Elizabeth

Cerâmica S.A.

The fickerts are assembled in a quick-fit system and in groups of six, symmetrically

arranged in the tangential polishing tool, from now on simply called polishing heads. The

surface of the abrasive contacting the plate to be polished exhibits a certain convexity, so that

only a strip of the block has an effective contact zone with the tile surface the during polishing

operation. Figure 7 shows a polishing head with the attached abrasives and as a detail, the

swinging motion that the abrasive block does.

Figure 7: Representation of the abrasives mounted to the polishing head. Detail of the swing

motion of the abrasive block. Author.

29

2.3.2 ABRASIVE WEAR AND MATERIAL REMOVAL ON CERAMIC MATERIALS

Some researchers consider polishing as a microscopic version of the grinding process,

where abrasive grains finely scratch the work surface and cut them. As with ceramic materials

generally the porcelain stoneware tile is very brittle and does not exhibit appreciable plastic

deformation. Nevertheless, due to the small penetration depth taking place during polishing,

even such brittle materials like ceramic stoneware tiles presents a prevailing ductile behaviour

(Sousa et al., 2014).

Whether by the action of silicon carbide or synthetic diamond, which is much harder

than ceramic tiles, the mechanism of wear and removal of material is the same. To better

understand this mechanism, it is necessary to approach from the perspective of the fracture

mechanics in the indentation process, which allows quantifying the brittleness index of the

ceramic materials, through the relation between hardness and toughness. From this perspective,

it is understandable to imagine that each abrasive grain behaves like an indenter being forced

to penetrate perpendicularly over the ceramic surface while being drawn parallel to the surface.

Thus, removal of the material can be schematized as a complex cracking system as shown in

Figure 8. The medium cracks are formed in the normal position to the surface during the normal

load application and the lateral cracks are developed parallel to the surface during the load

removal of the indentation cycle. The lateral cracks are responsible for the material removal

and the medium cracks for the residual damage during the penetration of the abrasive

(Sousa,2007a), (Huchtings, 1992).

Figure 8: Cracks system by the polishing of ceramic floor tiles. Author.

The removal rates during the polishing operation vary greatly depending on several

factors, such as abrasive grit, total polishing time, applied normal load, kinematics adopted, as

well as the microstructure of the material. The microstructure of the material has great influence

30

in reducing roughness and consequent gloss gain. Porcelain stoneware tiles that behave in a

ductile manner produce a better surface finish after polishing.

The behavior of brittle materials such as ceramic floors has been examined through

indentation tests, indicating that as the abrasive particle becomes smaller, the removal

mechanism changes from fracture to plastic flow. Therefore, the removal model is also a

function of abrasive grading. In the first grains, brittle removal is predominant, and it is

estimated that this mode prevails even between the granulometries #360 and #400, with a

change in the mode of removal after this stage. The transition from the brittle to the ductile

mode can be explained from material removal energy considerations. Thus, for smaller cutting

depths, the plastic flow is more energetically favorable, characterized by a less flow stress.

Another important factor, which affects the mode of removal, is the pores density present on

the polishing surface. Figure 9 shows the possible polishing mechanisms that occur during this

step, according to literature (Sousa et al., 2014).

Figure 9: Wear mechanism in porcelain stoneware tiles, as proposed by Sousa, 2014. Author.

2.3.3 INDUSTRIAL POLISHING OPERATION

High levels of gloss, of approximately 80 units of gloss, are achieved after the porcelain

stoneware tiles pass through a gradual sequence of 30 to 32 polishing heads positioned in series

and with decreasing sizes of abrasives, constituting a polishing line or polishing train

31

(Hutchings et al., 2005a), (Sousa et al., 2007b). Figure 10 shows an industrial polishing line

with a detail of the polishing head.

Figure 10: Industrial polishing train. Courtesy of Elizabeh Ceramic, S.A.

According to the rate of material removal, the polishing operation can be subdivided

into three sub-steps: grinding, semi-polishing and polishing (Wang et al., 2003) . One of the

most important factors in the polishing operation is the definition of the abrasive sequence.

Table 3 lists the number of abrasive grits, the average size of the abrasive particle and the

associated substep.

Table 3: Relation between grit number and the average size of the abrasive particles used in

the polishing operation. Author.

Subsptep Grit

Average

size

(µm)

Subsptep Grit

Average

size

(µm)

Subsptep Grit

Average

size

(µm)

Gri

ndin

g

#36 530.0

Sem

i-poli

shin

g

#320 29.2

Poli

shin

g

#800 6.5 #46 390.0

#60 270.0 #360 22.8 #1000 4.5

#80 190.0

#100 130.0 #400 17.3 #1220 3

#120 110.0

#180 75.0 #600 9.3 *Lux <3.0

#220 63.0

#280 36.5

*Generally grits between #1500 and #2500.

Diamond abrasive blocks, whose matrix is metallic, are generally used in the grinding

grits (#36 - # 280). Due to its high wear resistance, this type of abrasive has a relatively long

life, lasting between 90 and 96h The SiC abrasive in magnesian matrix has a variation between

15min, in the first grits, to 20h when used in the last steps.

32

Besides time, the kinematics adopted represents another important feature to be

considered in the industrial polishing operation. Using a radial motion polishing machine, the

main kinematic parameters are the angular velocity of the polishing head, and the forward speed

of the polishing train. With the technological advance of this sector, from the 2000s a new

generation of industrial polishers appeared with an extra movement available: the transversal

motion. It is a movement in which the polishing head oscillates horizontally and transversely

with respect to the polished train. Therefore, the control of the lateral oscillation movement

(period and amplitude) became another important variable to be considered in the industrial

polishing operation (Sousa et al., 2007a). In addition to minimizing gloss pattern defects, one

advantage of oscillating motion of the polishing head is the ability to polish larger pieces.

2.4 SCIENTIFIC BREAKTHROUGHS IN PORCELAIN POLISHING

The polishing as a function of time has been studied for a few years. One of the first to

investigate the glossiness gain over the polishing time was Ian M. Hutchings (Hutchings et al.,

2005b). He established important relationships for the performance of the polishing process in

relation to the glossiness gain and roughness reduction, which became known as Hutchings

equations for glossiness and roughness, equations 1 and 2, they are graphically illustrated as

exponential curves, as shown in Figure 11.

𝑅(𝑡) = 𝑅∞ + (𝑅∞ − 𝑅0). 𝑒−𝑡

𝑡𝑅 (1)

𝐺(𝑡) = 𝐺∞ − (𝐺∞ − 𝐺0). 𝑒−𝑡

𝑡𝑔 (2)

33

Figure 11: Typical asymptotic curves of gloss and roughness, as defined by Hutchings

(Hutchings et al., 2005b).

In terms of the geometry of the polishing head, it is quite noticeable that there is a lack

of abrasive material in the centre of the polishing head, so it is reasonable to imagine that the

glossiness patterns generated on the surface using the two types of polishing, radial, and

transverse are also different. The spatial distribution of surface glossiness was studied with a

pioneering approach by Vicente Cantavela (Cantavella et al., 2004) when he developed a

mathematical model to predict the behavior of the gloss distribution using a radial type polisher.

In 2007, Fabio J.P Sousa (Sousa et al., 2007b), using the kinematic equations involved in the

polishing process and considering the lateral oscillation movement, reached another gloss

pattern.

According to Cantavela, due to the absence of abrasive material at the centre of the tool,

the abrasive particles closest to the centre of the polishing head exert a higher contact pressure

than the more peripheral ones. Thus, using only the radial movement polisher, it is

understandable that a marked gloss occurs in the regions near the boundary of the area covered

by the lacking of abrasive material. At the centre of the polishing workpiece and the respective

peripheral regions (where a lower contact pressure of the abrasive particles occurs), a defective

polishing and less removal of material occur. In the second case, a zig-zag type of gloss pattern

is generated on the surface due to the oscillatory movement of the abrasive tool, as well as the

overlapping of the scratches in the regions of greater abrasive contact and in regions absent

from the center of the polishing head. Figure 12 shows the difference between the patterns

found by Cantavela and Sousa (Cantavella et al., 2004) (Sousa et al., 2007b).

34

Figure 12: Radial motion (a) and transverse motion (b) with respective generated glossiness

patterns. Adapted from Cantavela, 2004 and Sousa, 2007b.

From 2007, the research on the process of polishing porcelain tiles was seen by another

perspective by Sousa (Sousa, 2007a), when in his thesis the path of a single abrasive particle

was calculated (Sousa et al., 2007c), through an analysis of the micro cinematics involved in

the process. In addition, a software was developed that simulated the polishing process in

accordance with the main technical parameters available in industrial polishing.

Based on the original knowledge generated by this thesis, it was possible to develop

many other studies aimed at evaluating the surface gloss distribution, considering the oscillatory

movement (Sousa et al., 2007d), as well as the distribution of surface time (Sousa et al., 2007b).

In 2008, Sousa (Sousa et al., 2008) discussed the distribution of gloss on the surface of

the porcelain stoneware tiles as a function of the adopted cinematic. Using the main variables

practiced in the industrial process in the computational modelling as a fundamental tool in the

understanding of this distribution of brightness, obtained a reasonable similarity with the actual

values found in the literature. Already in 2009, Sousa (Sousa et al., 2009), using computational

algorithms, simulated varied conditions of lateral oscillation frequency and velocity of

advancement. Once different conditions were used in the simulation, different distributions of

polishing time were obtained, so that it was possible to evaluate the influence of the polishing

35

kinematics on the spatial distribution of abrasive contacts. This study served as a guide for

further research in order to avoid excessive polishing and regions of glossiness gradients.

In view of the different glossiness patterns, the influence of the glossiness, over the

sliding resistance was also studied (Sousa et al., 2010a). Through the direct measurement of the

gloss and the friction coefficient of the surface after the polishing step, the results showed even

in regions of greater surface gloss, approximately 75 GU, no friction coefficient was found for

regions large enough to provide slippage. As a conclusion, the correlation obtained was not

strong enough to promote the establishment of the most recommended glossiness limits to be

aspirated by the industries.

During the process, scratching speed and the contact pressure are strongly related and

are both important factors influencing the evolution of the surface quality during polishing (Sani

et al., 2016). There is a difference in scratching speed between innermost and peripheral

abrasives particles due to rotation motion and the geometry of the fickerts (Nascimento and

Sousa, 2014), leading a pressure gradient along the radial direction of the fickert. According to

Sani (Sani et al., 2016), such pressure gradient tends to cause an inclination of the abraded

surfaces, becoming s table after a given polishing period.

The computational simulation has become an important tool in the improvement and

continuous optimization of the polishing process, facilitating the understanding of the

scratching process and the kinetics involved in the polishing of commercial porcelain stoneware

tiles (Sousa et al., 2013).

2.5 REUSE OF WASTES IN THE CERAMIC MATRIX

The world has been facing a considerable increase in industrial activities, starting with

the activities of the extractive and processing industries, which supply raw materials and

products for other industries, such as durable and non-durable products. Another activity that

is constantly growing in the world is that of civil construction, being more accentuated in some

countries and less expressive in others, but always growing. It is notable that with the increase

in these industrial activities the demand for natural raw materials and subsequent processing

increase as well. Another known factor is that natural resources are depletable and non-

renewable, and in this respect, the world is facing a time of imminent reduction in the supply

of raw materials, which can lead to the failure of natural deposits.

Over the past few years, much of the perceived and growing concern for the

environment, sometimes imposed by severe environmental laws that penalize those who

36

disregard them, have grown interest in finding alternative routes to the disposal of mineral and

industrial waste.

In general, the ceramic industry (cement, bricks, blocks, tiles and cladding) is considered

versatile from the point of view of solid waste management (Andreola et al., 2016). It is a waste

generator, but on the other hand, it is able to reuse a wide variety of wastes from other industrial

activities as an alternative material, additive or even as main raw material (Kazmi et al., 2016).

Generally in this production sector, all the waste generated before the heat treatment returns to

the initial processing as filler material, after the heat treatment, practically all the waste

generated is sent to other industrial activities, such as construction, used in the manufacture of

cement, (Penteado et al., 2016), bricks, concrete and landfill (Halicka et al., 2013). This

important role played by the ceramic industry in waste recycling is due to there are similarities

in terms of chemical and mineralogical compositions between many mining-industrial wastes

and conventional raw materials.

In this way, some fruitful works have been developed in order to add value to waste and

reduce the use of natural sources. Therefore, the most varied types of wastes have been studied,

from different activities such as mining, industrial and agroindustrial. All the investigations

aimed to attempt to explain how the insertion of residues into the tile ceramic compositions

influences the technological properties of the ceramic floor tiles and whether the technological

performance can vary according to a particular characteristic of each alternative material used.

De Oliveira has investigated in his thesis, the insertion of wastes from the kaolin and

emerald beneficiation, shards resulting from the breakage of red ceramic bodies and vegetable

ash in ceramic floor tiles compositions. After sintering, the developed formulations were

classified in all types of ceramic floor tiles (from porous to porcelain stoneware tile), according

to the Brazilian standard NBR 13817, with water absorption rates varying of 0.0% to 27.97%

and flexural strength of 6.90 to 51.76 ± 4.62 MPa (de Oliveira, 2012).

In a recent study conducted by Acchar (Acchar et al., 2013), it was proved the feasibility

of incorporating coffee (untreated) ashes as an alternative raw material to feldspar, the main

fluxing agent used in the production. After additions of 5% to 20% in clay-based compositions

and based on results obtained for the technological properties (WA, FS and LS), guided by EN

176 and NBR 13817, was concluded that the mixture of 10 wt.% of coffee ashes added to the

clay mix, treated at 1180 ° C, best meets the standards mentioned, and does not require

significant changes in the processing parameters.

Volcanic ash when deposited results in serious problems in urban and agricultural areas

and was investigated by Serra (Serra et al., 2015). Ash samples from the western coast of Lake

37

Traful, Argentina, were characterized and evaluated the applicability in the manufacturing

process of ceramic materials based on clay, according to the triaxial diagram of ceramics. The

volcanic ash investigated showed a silicon-aluminous composition (70.43 wt.% - SiO2, 15.03

wt.% - Al2O3) similar to the potassic feldspar traditionally used in the production. The sintering

and the evolution of the mechanical properties of the produced ceramics were also studied and

compared with a triaxial composition. It was demonstrated that, with thermal cycles similar to

those used for the manufacture of traditional ceramics, the ceramics obtained from the use of

the volcanic ash of Lake Traful had adequate textural and mechanical properties, comparable

to the materials produced with the traditional raw materials.

Gouvêa and others (Gouvêa et al., 2015), studied the potential for the implementation

of bovine bone ash in small quantities of (1 to 5 w.t%) in ceramic-based kaolinitic products

according to the ceramic triaxial diagram and varying the sintering temperature between 1000

and 1400 °C in order to evaluate the level of sintering, the microstructure and the technological

properties. As the main result, it was observed that the addition of 2 wt.% of bovine bone ash

accelerates the formation of mullite and for the addition of 5 wt.%, the formation of liquid phase

decreased the initial sintering temperature. For the additions, only the liquid phase, mullite and

residual quartz were observed by XRD.

An investigation guided by Pinheiro and Holanda (Pinheiro and Holanda, 2013) showed

the reuse of solid wastes from the oil industry as a complement of kaolin in porcelain stoneware

tiles formulations fired at 1240 ° C. Three different substitutions, 1.25, 2.5 and 5 wt.% were

tested. Based on the results of WA and FS, it was concluded that the concentrations of 1.25 and

2.5 wt.% was possible to obtain porcelain stoneware tile. In addition to the main tests, this study

presented a leaching toxicity analysis for heavy metals (Ag, As, Ba, Cd, Cr, Hg, Pb) of all

formulations, which showed that the presence of these metals in very low concentrations, well

below the limits established by current regulations, do not cause serious risks to the

environment.

Glass waste from LCD panels was well applied as a flux material substituting for the

feldspar by Kim and others (Kim et al., 2015). As a result and conclusion, substitutions of up

to 87 wt.% of the feldspar replacement, no pyroclastic deformations and liquid exudation were

evidenced. The content of mullite remained practically unchanged and the coefficient of

thermal expansion and water absorption properties were positively influenced by the use of the

LCD glass waste. In this follow-up, other studies have been elaborated satisfactorily with the

use of glass rejects as an alternative raw material for several ceramic products (Soares Filho,

2013), (Soares Filho et al., 2014), (Dondi et al., 2009).

38

A study was conducted in Malaysia by Teo (Teo et al., 2014), in which electric arc

furnace slag waste was added to ceramic floor tiles formulations and compared with

commercial floor tiles. The electric furnace slag used was chemically characterized and showed

high Fe2O3 (31.70 - 32.52 wt.%), SiO2 (19.73 - 20.50 wt.%), CaO (29.00 - 29.45 wt.%) and

Al2O3 (8.83 - 8.58 wt.%). They introduced 40, 50 and 60 wt.% of such wastes with different

proportions of the conventional raw materials, clay, quartz, and feldspar. As a result, as they

slag was added up to 60 wt.%, greater apparent porosity and water absorption were observed,

accompanied by a reduction in bulk density as well as a reduction in flexural strength. Thus,

the best result was the incorporation of 40 wt.% of slag, contributing to the excellent apparent

porosity, water absorption and flexural strength of the ceramic floor tile when compared to

conventional ceramic floor tiles.

Metallic waste from a stainless steel plant, well known as steel dust, was studied by

Zhang et al. (Zhang et al., 2014). Although it was considered harmful due to the presence of

heavy metals in its composition, the residue was used as a colouring agent in ceramic tile

pigmentation. The pigments were prepared with mixtures of 50, 60, 70, 80, 90 and 100 wt.%

with commercial Cr2O3, then added to the working ceramic mixture. Cylindrical ceramic bodies

were carried out with different proportions of the pigment incorporated (0, 2, 4, 6, 8, 10, 20

wt.%) and sintered at different temperatures (1100, 1150, 1175 and 1200 °C). The results of the

tests of compressive strength of the ceramic cylinders exceeded the minimum limit imposed by

the Chinese standard for resistance to the compression of polished porcelain stoneware tile. The

leachable toxic substances also reach the limits established by the laws of that country.

The waste originated from the own porcelain tiles polishing process has already been

studied as an ecologically and low-cost raw material in the production of porcelain tiles. Which

means that is the near future may represent a totally clean and sustainable production process,

according to Shanjun et al. (Ke et al., 2016). Was evaluated the feasibility of incorporating this

residue into the alternative raw material in the porcelain production. The X-ray diffraction

pattern of the residue indicated that the material might influences in the mullite development

during thermal treatment. Seven mixtures were checked, from 10% up to 70% (w.t %) of

incorporation. The technological properties (linear shrinkage, water absorption, and bulk

density) were evaluated, as well as the mechanical performance. The composition with 50% of

such waste included, fired at 1120 °C, represented the best result within the requirements of

ISO 13006, presenting with water absorption of 0.12%, bulk density of 2.49 g/cm3 and flexural

strength of 47 MPa.

39

The influence of some factors such as raw materials traditionally used in production,

residues incorporated as substitute materials and process parameters directly affect the

development of mullite in the final microstructure, as well as the existence of porosity

(interconnected or not) and microcracks after heat treatment (Pérez et al., 2012). As

consequence, the final properties (water absorption, linear shrinkage, flexural strength, acoustic

and thermal properties) are also influenced (Romero and Pérez, 2015), (Rambaldi et al., 2015).

As seen in this session, there are several types of residues being studied for potential

incorporations in formulations of ceramic tiles. However, almost all of them are focused in the

investigation of the technologic and microstructural properties. This topic encourages the idea

of the need for studies that evaluate the superficial characteristics of porcelain stoneware tiles

conceived from the use of residues as alternative raw materials.

40

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45

3 RESULTS AND DISCUSSION

THESIS STRUCTURE

The present thesis is structured in four chapters showing the main results of the

experiments. Each one stands for one or more of the specific objectives proposed in the thesis.

Chapter 1, High-performance “green” ceramic tiles formulations with industrial

wastes: development and microstructural evaluation, was designed to assess the capability

and introducing mineral and industrial wastes from the state of Rio Grande do Norte - Northeast

of Brazil, as alternative raw materials in manufacturing polished porcelain stoneware. The

investigation consists of analysing the selected wastes (chemical, mineralogical and thermal

analysis), developing the porcelain stoneware tiles according to the current technical standards,

and establishing relationships between microstructure and performance (mechanical and

surface) of the compositions. This investigation addresses the goals 1 " To conceive

formulations with porcelain stoneware tile characteristics using of raw materials originating

from the Rio Grande do Norte, incorporating industrial wastes” - and 2 – “To characterize

formulations in technological and mechanical properties terms, in order to attest to the

technical feasibility of obtaining porcelain stoneware tile with the residues” of the thesis.

In chapter 2, Estimation of the minimum material removal thickness during the

polishing process of ceramic tiles by laser triangulation, the main goal of this study is to use

laser triangulation technique as a pre-polishing process step, in order to verify the potential use

for future devices. For this purpose, different surfaces of porcelain tiles made with wastes in its

composition were 3D mapped, and their waviness profiles and surface roughness were

determined. This investigation addresses the goal 3 – “Obtaining a superficial analysis by 3D

mapping at a pre-polishing step of the samples” of the thesis.

In the chapters 3 - Polishing performance of eco-friendly ceramic tiles made with

wastes from bricks and roof tiles, and 4 - Evaluation of surface quality after polishing of

eco-friendly porcelain stoneware tiles made with kaolin ore waste, the influence of two

different industrial wastes on the microstructure and surface quality after the polishing,

respectively was actively analyzed. The investigations focused on evaluating the polishing

performance, by checking of the gloss and roughness curves, as well as the microstructure and

topography differences of the eco-friendly porcelain stoneware tiles. The investigation

addresses the goals 4 – “Obtaining the curves for gloss and roughness performance throughout

the abrasive sequence” and 5 – “Establish relationships between the microstructures and the

glossiness gain” of the thesis.

46

3.1 HIGH-PERFORMANCE “GREEN” CERAMIC TILES FORMULATIONS

WITH INDUSTRIAL INORGANIC WASTES

*(Submitted to the Journal of Cleaner Production)

This chapter investigates the potential of using industrial inorganic wastes as alternative

raw materials in the manufacture of polished porcelain stoneware tiles. Such products have

great aesthetic values, high-performance properties, and high demand worldwide, but is still

associated with high production costs. This investigation divided into two parts: the first one

focuses on the characterization of the wastes (chemical, mineralogical and thermal behaviours).

The second part focuses on the development of ten compositions with the traditional raw

materials and microstructural evaluation. Five different types of mineral wastes were tested,

and they were obtained from the first and final stages of kaolin beneficiation process, fine

powders from the crushing processing of granitic rocks, from the extraction and processing of

scheelite mineral, and finally from shards of red ceramic bodies (bricks), colloquially known

as grog. When compared to the raw currently consolidated materials in the production of

porcelain stoneware, such as clays, feldspar, and quartz, all wastes presented similar chemical

and mineralogical compositions, as well as thermal behaviour. The results obtained for the ten

developed compositions show that they fall within the range specified by floor tile standards

(NBR 13818 and EN 176), with just one composition requiring a change in processing

parameters and the others, advantageously, did not require significant changes in the respective

parameters. Thus, with replacements of up to 20% of natural raw materials for industrial wastes,

this research contributes to the clean production of a high-performance ceramic product, adding

value to the wastes that are usually discarded.

Introduction

The exploitation of natural resources is accountable for the production of large

quantities of wastes. In Brazil, several mounds of inorganic wastes can be found disposed along

the main roads and in some cases without considering their environmental impacts. The

increasing industrial demand for such raw materials leads to more intense exploitation activities

[1]. Consequently, the generation of rejects from mining and other industrial activities also

increases. Nevertheless, the interest in reusing them increases at the same rate [2][3][4][5].

47

Many inorganic wastes contain a relevant percentage of silica and other oxides; their

compositions could be comparable to those of raw materials traditionally used in porcelain

stoneware tiles and glass ceramic manufactures. In general, the ceramic industry is considered

flexible and versatile under the ecological point of view. Is a waste generator but, on the other

hand, it is also capable of reuse a large variety of residues as additives or alternative raw

materials. Nowadays the ceramic sector plays an important role in the reuse of inorganic wastes.

In some cases, wastes and conventional resources have great similarities both in chemical and

mineralogical compositions [1][6], and several studies have reported the incorporation of

industrial wastes in the original formulation of various products, such as cement [7], sealing

blocks (bricks) [8], roof tiles [9], coverings, and also porcelain stoneware tiles

[10][11][12][13][14].

Porcelain stoneware tiles stands out as the most advanced product among the ceramic

tiles. Its industrial production has been growing rapidly since the beginning of this century

[15][16][17]. Such tiles present high surface hardness, mechanical strength, water absorption

(≤ 0.5%) [18] and low superficial porosity, and a microstructure that provides high resistance

against chemical attack and staining [19][20][21]. Besides, the aesthetic value of such tiles can

be greatly enhanced when they undergo a polishing process to achieve high glossiness [22].

This paper was designed to assess the capability and introducing mineral and industrial

wastes from the state of Rio Grande do Norte, Northeast of Brazil, as alternative raw materials

in manufacturing polished porcelain stoneware. The investigation consists of analysing the

selected wastes (chemical, mineralogical and thermal), developing the porcelain stoneware tiles

according to the current technical standards, and establishing relationships between

microstructure and performance (mechanical and superficial) of the resulting compositions.

Experimental

Five different types of wastes from mining and industrial activities in the state of Rio

Grande do Norte - northeastern Brazil were investigated:

Coarse waste from kaolin beneficiation process – CK

Fine waste from kaolin beneficiation process – FK

Waste from the extraction of granitic rocks for civil construction (finely crushed powder

colloquially known as “granitic powder”) – GP

Waste from processing and beneficiation of calcium tungstate mineral (CaWO4) – CW

48

Shards resulting from the breakage of red ceramic bodies, mainly tiles and bricks

(colloquially known as “grog”) – TB

After collecting the wastes in situ, samples were prepared and milled to obtain a particle

size smaller than 200 mesh (74μm), for characterization purposes. The determination of

chemical composition was performed by X-ray fluorescence spectroscopy (XRF) with a S2

Ranger Bruker equipment. The mineralogical composition was determined by the technique of

X-ray diffraction (XRD) on a Bruker D2 Phaser apparatus using CuKα radiation (λ = 1.54056Å)

with a Ni filter, with 0.02º step, 10 mA, 30kV voltage and using a Lynx-eye detector. The

crystalline phases were identified by the JCPDS-ICDD diffraction file. The thermal behaviour

of the wastes was obtained by thermogravimetric (TG) and differential thermal (DTA) analyses,

using a thermal differential DTA 60 analyser from Shimadzu, under an argon flow of 50 ml /

min, with a heating rate of 10 °C/min between 25 °C and 1000 °C.

Supported by the results of the characterizations were added between 5 and 20 wt.% as

an alternative raw material in 10 different compositions, 2 mixtures for each waste, as can be

seen in Table 4.

Table 4: Compositions developed.

Compositions Main raw materials (wt.%) Wastes (wt.%)

Clay Feldspar Quartz CK FK GP TB CW

CK1 45 40 - 15 - - - -

CK2 41 39 - 20 - - - -

FK1 49 46 - - 5 - - -

FK2 37 48 10 - 5 - - -

GP1 42 40 8 - - 10 - -

GP2 42 40 4 - - 14 - -

TB1 40 50 - - - - 10 -

TB2 40 35 10 - - - 15 -

CW1 42 50 - - - - - 10

CW2 35 35 10 - - - - 20

The formulations were wet-milled (1h) in a porcelain jar with alumina balls and 1% de

sodium silicate as a de-flocculant additive, obtaining 60% of solids content, and then dried at

110 °C for 24 hours. After drying, they were passed through a 35-mesh sieve. Then 8 wt.% of

water was added to assist the compaction in a hydraulic press with 45 MPa (uniaxial pressure).

49

Two different rectangular sizes of work pieces were produced: 60 mm x 20 mm and 115 mm

by 55 mm, after the production the work pieces were dried at 110 °C for 24 hours again. The

work pieces were submitted to heat treatment in electric oven, under a heating rate of 10 ° C /

min, 20-minute sintering time at a final temperature of 1250 ° C. The cooling was in a natural

condition. These parameters aim to provide adequate conditions for obtaining specimens with

a higher rate of sintering and densification.

The two main criteria used to classify the floor tiles as porcelain stoneware tile are the

percentage of water absorption (WA) and mechanical performance by flexural strength (FS). In

this study both properties were properly characterized in accordance with NBR 13818 [23] and

EN 176 [24] standards.

The microstructures of the specimens after the heat treatment were observed on fracture

surfaces after chemical etching with hydrofluoric acid (HF) 2 vol%. During SEM observations,

the chemicals compositions of the phases were qualitatively accessed by energy dispersive X-

ray fluorescence spectrophotometry (EDX).

Results and discussion

Table 5 shows the chemical analyses performed on the waste samples. The samples of

coarse and fine kaolin showed typical characteristics of each stage of the crude kaolin treatment.

Coarse kaolin is the first waste generated during the beneficiation process. It was detected a

high percentage of silicon oxide, 64.69 wt.%, typical of this stage. In the fine Kaolin, the coarse

material already was removed by sieving. Therefore, the amount of silicon oxide found was

smaller (52.89%). The differences between the amounts of silico-aluminous materials and the

quantities of fluxes oxides (K2O, CaO, MgO, and Na2O) became evident when comparing these

two types of waste.

Table 5: Chemical analysis of the wastes used, as determined by XRF.

Wastes Oxides (wt.%)

Total SiO2 Al2O3 Fe2O3 K2O CaO MgO TiO2 Na2O *Traces **L.I

CK 64.99 14.98 3.65 5.63 3.34 0.97 0.53 2.53 0.60 2.78

100 w

t.%

FK 52.89 34.13 0.57 1.33 0.04 0.12 0.04 0.17 0.21 10.50

GP 67.25 15.12 2.71 6.42 2.78 - 0.42 3.97 0.61 0.72

TB 54.96 22.51 8.94 3.4 1.57 3.12 1.02 1.66 0.42 2.40

50

CW 19.33 5.42 3.52 0.66 42.94 2.33 0.29 0.23 0.46 24.82

* Traces = Total sum of oxides with values less than 0.1%.

** L.I = Loss on ignition, 1000°C.

In terms of chemical composition, the granitic powder also showed a desirable similarity

to the main raw materials used in the production of porcelain stoneware, 67.25 wt.% SiO2,

15.12 wt.% Al2O3, 2.73 wt.% Fe2O3, and moderate proportions of oxides liquid phase formers,

with 6.42 wt.%, 2.78 wt.%, 3.97 wt.% and 0.42 wt.% for K2O, CaO, Na2O and TiO2,

respectively.

The waste from the breakage of red ceramic bodies (grog) is usually capable of

providing a structural function in porcelain stoneware compositions for producing floor tiles.

In fact, it has a considerable amount of silicon oxide (54.96 wt.%), so that the quartz is

responsible for structural control. However, it also showed a total of 10.78 wt.% of fluxes

oxides, indicating a potential to be used as alternative flux. A total of 8.94 wt.% of Fe2O3 was

found in the sample, indicating that a red coloration is expected after the heating treatment. The

intensity of the red coloration will vary depending on the amount of waste used. This behaviour

may limit the maximum amount of grog that could be introduced into the tile formulations,

when such variation in collor is not a desired characteristic.

Regarding the waste from the extraction of calcium tungstate ore (CW), the chemical

analysis indicated a high efficiency in the beneficiation process, since no traces of WO3

(tungsten oxide) were found in the sample. Besides presenting a high percentage of ignition

loss (24.82 wt.%), it must be pointed out that this waste presents almost half of the total

chemical composition of oxides of flux, equivalent to 46.22 wt.%, of which 42.94 wt.% is CaO,

responsible for promoting the formation of the liquid phase in porcelain stoneware.

Figure 13 presents the X-ray diffraction patterns. As expected from the XRF results, the

mineralogical phases of the residues derived from the treatment of kaolin waste (coarse and

fine). Accordingly, it showed kaolinite and quartz as main phases, as well as others peaks fewer

intense. The kaolinite peaks are more intense in fine kaolin than in coarse kaolin, in contrast,

the coarse waste presented higher quartz peaks than in fine waste.

The diffraction pattern for the rock powder shows the mineralogical phases with flux

characteristics, such as microcline and albite, and was possible to detect quartz.

The X-ray diffraction pattern presented by the grog, is comparatively much noisier. But

as before, the main identified phases were detected at well-defined peaks, indicating the

presence of quartz, albite and ilite.

51

The waste from the extraction and beneficiation of calcium tungstate consists of calcite,

anorthite, cordierite and quartz. The most intense reflection refers to calcite, which is caused by

the presence of 42.94 wt.% CaO, in good agreement with the XRF results (Table 5). The other

phases peaks show less intensity than the peak of calcite.

Figure 13: X–ray diffraction patterns of the wastes.

Thermogravimetric curves (TG) allow evaluating the total weight loss and the

temperature range in which occurs the greatest loss of mass of each studied waste. The

differential heat (DTA) curves provide the detection of endothermic and exothermic peaks for

each event recorded during the thermal analysis.

The thermogravimetric analysis of CK reveals in Fig 14 a total of 2.78 wt.% of mass

loss. The highest weight loss (1.81%) was identified between the range of 430.08 °C and 694.62

°C, due to two endothermic events identified in the DTA curve at temperatures of 528 °C and

580.21 °C, respectively regarding the dehydroxylation of the kaolinite (kaolinite →

metakaolinite) and quartz inversion (α → β). The smoothing of the peak of the dehydroxylation

of kaolinite may be associated to both low kaolinite content, as shown by the qualitative analysis

52

phases, and to kaolinite with structural defects. The conversion of metakaolinit into primary

mullite is an exothermic transformation and occurred at 993.20 °C.

Figure 14: Thermogravimetric and differential thermal analyses of CK.

In figure 15, the thermal analysis of FK, a total of 10.50 wt.% of the loss of mass was

found for the total analysis range. The interval between 446.28 °C and 692.15 °C represents

8.73% of the total weight loss. At temperatures between 528.11 °C and 582.20 °C, there was a

junction of peaks forming an endothermic band referring to the dehydroxylation of kaolinite

and quartz inversion. This confirms the quantitative result of crystallographic phases presented

for this waste, which consists of kaolinite and small quantities of quartz. It is also noticed a

change in the DTA curve at 983.67 °C indicating the conversion of metakaolinit into primary

mullite.

53

Figure 15: Thermogravimetric and differential thermal analyses of FK.

Figure 16 shows the thermal analysis GP. An excellent relative thermal stability was

verified, with a total mass loss of only 0.72 wt.%. For the temperature range used, the main

components of this material (albite and microcline) are thermally stable. The thermal events

recorded are assigned to quartz and a small organic fraction. During the temperature range from

562.24 °C to 676.45 °C occurs a deflection in the thermogravimetric curve. This behaviour is

caused by liberation of organic compounds and inversion of quartz (α → β), but it was less

pronounced in the DTA curve due to the small amount of quartz present in the sample.

Figure 16: Thermogravimetric and differential thermal analyses of GP.

54

The waste TB is originated from products usually submitted to a heat treatment between

800 to 900 °C. Therefore, a stable thermal behaviour was to be expected. However, according

to the thermal analysis showed in Figure 17, this material still showed a total loss of mass of

2.40 wt.%, such behaviour could be attributed to an inefficient control during the thermal cycle

(heating rate and final temperature) in small factories. This lack of knowledge in this treatment

causes defects by trapping organic matter, commonly known as "black core".

Figure 17: Thermogravimetric and differential thermal analyses of TB.

In Figure 18 is possible to verify that the thermal behaviour of waste CW was

completely different from the others, on the other hand, entirely coherent with the XRD and

XRF analysis. Nearly the entire loss on ignition of 24.82 wt.% occurs between 588.35 °C and

762.74 °C. It was found an endothermic peak at 742.53 °C related to decarbonisation of calcite

(CaCO3), the main and the most thermally unstable constituent of this waste, due to the release

of CO2 at this temperature.

55

Figure 18: Thermogravimetric and differential thermal analyses of CW.

The characterization techniques used herein were altogether consistent to each other,

and the results testify the feasibility of incorporating the residues into new porcelain stoneware

formulations. The main phases found in the residues are similar to most of the raw materials

traditionally used in industry. Based on these characteristics, it was possible to manage the

incorporation of these alternative materials almost directly, either by partially replacing clay,

feldspar, or, in some formulations, by replacing all the conventional source of quartz, which is

usually only a complement to the quartz already contained in the others raw materials.

Based on the discussion before, the major technological properties, namely water

absorption (WA) and flexural strength (FS) were evaluated. The performances of each

composition can be related to the amount of waste incorporated and are showed at the graphs

of Figure 19.

The graph (a) in the Fig. 19 shows the WA for the compositions. All compositions are

clearly in the WA range specified in the standards. According to the standard EN 176 [24], the

WA needs to be below 3.0% to be considered as porcelain tiles. In contrast, the NBR 13818

[23] requires a level of WA below 0.5% to be considered as porcelain stoneware tiles.

For the Mixtures 1 and 2, using respectively 15 wt.% and 20wt.% of coarse kaolin in

substitution to the quartz, it was possible to keep unchanged the average value WA (0.09%).

This shows the potential for replacement without changing the average WA. For formulations

3 and 4, with fine kaolin, the variation in WA was small, only 0.04 wt.% between the two

formulations, with composition 4 having the least percentage of clay in the mixture.

56

The compositions 5 and 6 presented WA below 0.1%, reaching zero level for the

composition 5. This can be related to the flux agent content shown by the powder of granitic

rocks, with two flux phases (microcline and albite).

It is very important to mention that the waste CW is rich in calcite, as could be seen in

Table 5 and Figures 13 and 18. Calcite has an abrupt decomposition that may be inappropriate

in some formulations, depending on the heating rate to be used. Due to the absence of studies

about the introduction of this type of residues in porcelain tiles compositions, it was decided to

test two compositions (9 and 10 in Table 4), with substitutions in the quartz and feldspar

amounts. In formulation 10 an amount of 20 wt.% was used, which culminated in pyroplastic

deformation at this temperature, as can be seen in the graph (a) of Figure 3. New work pieces

were made of the same composition but this time using 1220 ° C as threshold temperature. WA

values were satisfactory for both compositions, although the FS value (Figure 19, graph b) of

formulation 9 (10% waste) not followed the recommendation of the Brazilian standard. This

behaviour can be explained by the prevailing of interconnected pores, which causes brittleness.

57

Figure 19: Water absorption (a) and flexural strength of compositions.

The flexural strength is related to the composition, size and morphology of the grain

material and to presence of faults. The mechanical performance of the compositions studied

here can be explained as a function of the wastes used, as well as taking into account the

different microstructures developed during the firing process, originated from the mixture of

58

conventional and alternative raw materials. All blends studied here will show good mechanical

performances, with the exception of composition 9, already mentioned above.

Fig. 20 shows precise microstructural details of the compositions developed with the

wastes from the kaolin beneficiation process (Fig. 20a for the composition 2 and Fig. 20b for

the composition 4). Both images show the typical morphology of secondary mullite needles

(region 1), quartz grain (region 2) and closed porous (indicated by *). Micro-cracks were also

observed in both compositions, and according to [4] [25], it can be directly attributed to

differences in coefficient of thermal expansion between microstructure phases during cooling

stage. Phases with different coefficient of thermal expansion can be cooled at slightly different

rates, resulting in micro-cracks.

The composition 4, with 5 wt.% fine kaolin (Fig. 20b), shows a comparatively dense

structure and with more areas of mullite than the mixture 2, with 20 wt.% coarse kaolin. This

is due to the fine kaolin used, which showed a higher peak of kaolinite than the coarse kaolin

(Fig. 13).

Figure 20: Microstructures (as seen on SEM) of porcelain stoneware tiles obtained from: (a) composition 2, (b)

composition 4. Details showing mullite needles (1), quartz grains (2), porous (*) and micro-cracks.

59

As can be seen in Fig. 21 and 22, common features of the ceramic tiles microstructures

were found for compositions 6 and 8, with rock powder and grog incorporated, respectively.

The wastes were able to contribute, in the amounts used, to the microstructural development in

the formulations evaluated as the development of mullite, formation of flux and provision of

quartz required.

Figure 21: Microstructures, as seen on SEM, of porcelain stoneware tiles obtained for composition 6. Details

showing mullite needles (1), quartz grains (2) e porous (*).

Like the others SEM images, both images show the typical morphology of secondary

mullite needles (region 1), quartz grain (region 2) and closed porous (*).

Figure 22: Microstructures, as seen on SEM, of porcelain stoneware tiles obtained for composition 8. Details

showing mullite needles (1), quartz grains (2) e porous (*).

In Fig. 23, the microstructure of the composition of 9, with 10 wt.% of the scheelite

residues included, showed a different structure from the others. In addition to the typical

morphology of secondary mullite needles (region 1), quartz grain (region 2) and closed porous

(*), regions with interconnected pores (3) as a “ceramic foam” were found. The “foam” is a

signal of calcite decomposition during the heat treatment.

60

Mixtures that contain irregular interconnected pores (internal) probably are results from

an insufficient densification and can result in a negative influence on the flexural strength. In

fact, the composition 9 based on the mechanical performance, is in accordance only with EN

176 [24] and not with NBR 13818 [23], also considered in this work.

Figure 23: Microstructure (as seen on SEM) of porcelain stoneware tiles obtained for composition 9. Details

showing mullite needles (1), quartz grains (2), “ceramic foam” (3), and porous (*).

A semi-qualitative analysis by EDX showed the presence of Al, Si in higher proportions,

and Na, K, Ca, and Fe in smaller proportions for the region 1 and practically only Si for the

region 2, of all pictures (all compositions). This result could confirm the presence of mullite

and quartz mixtures in the vitreous matrix.

Conclusions

The folowing conclusions can be drawn:

- For the wastes from kaolin beneficiation process, between 5 and 20 wt.% may be

successfully added. CK has similar characteristics with that of the quartz, being able to fully

replace the additional amounts contents that the composition needs. On the other hand, FK has

characteristics like the benefited kaolin, being able to be used as substitute (partial) raw material

of clays or a plastic additive of the formulation.

- The GP proved to be an excellent alternative raw material, optimizing the

densification, promoting an increase in the mechanical strength, reducing to the lowest WA

level.

- The TB has shown that it can also be used as an alternative source of quartz, and its

present liquid phase forming potential, up to 10 wt.%. In amounts up to 20 wt.% it is

recommended to combine the substitution of quartz with the minor substitution of feldspar.

61

- The CW was not suitable for amounts greater than 10 wt.% for the temperature used,

with a high pyroplasticity observed. However, it has potential to be a fluxing agent. A more

careful study of this waste should be done in order to clarify and improve its efficiency in the

process of temperature reduction, as well as the relationships with the calcite content.

Thus, this work demonstrates that these wastes of mining and industrial activities from

Rio Grande do Norte - Brazil can satisfactorily replace the main raw materials (feldspars, clay,

and quartz). In the percentages carried out here, they are capable contributing to the formation

of the final microstructure as a source of mullite (fine kaolin contributing higher amount than

coarse kaolin), the liquid phase (grog, rock powder and scheelite) and inert material (coarse

kaolin and grog showing higher contribution than the others do). In addition to contributing to

the reduction of environmental impact, this study adds value to the residues studied, which are

abundant in the mentioned region.

References

[1] F. Andreola, L. Barbieri, I. Lancellotti, C. Leonelli, and T. Manfredini, “Recycling of

industrial wastes in ceramic manufacturing: State of art and glass case studies,” Ceram.

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Ferreira, “Utilization of kaolin processing waste for the production of porous ceramic

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[5] R. M. Novais, M. P. Seabra, and J. A. Labrincha, “Wood waste incorporation for

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[6] R. R. Menezes, G. A. Neves, J. Souza, W. A. Melo, H. S. Ferreira, and H. C. Ferreira,

“Atividade pozolânica dos resíduos do beneficiamento do caulim para uso em

argamassas para alvenaria,” Rev. Bras. Eng. Agrícola e Ambient., vol. 13, no. 6, pp. 795–

801, Dec. 2009.

[7] H. Elçi, “Utilisation of crushed floor and wall tile wastes as aggregate in concrete

production,” J. Clean. Prod., vol. 112, 2016.

[8] G. Nirmala and G. Viruthagiri, “A view of microstructure with technological behavior

of waste incorporated ceramic bricks,” Spectrochim. Acta - Part A Mol. Biomol.

Spectrosc., vol. 135, pp. 76–80, 2015.

62

[9] M. Dondi, G. Guarini, M. Raimondo, and C. Zanelli, “Recycling PC and TV waste glass

in clay bricks and roof tiles,” Waste Manag., vol. 29, no. 6, pp. 1945–1951, Jun. 2009.

[10] F. H. G. Leite and T. F. Almeida, “Caracterização de chamote e casca de ovo para

produção de material cerâmico,” in I encontro de engenharia, Ciencia de Materiais e

inovação do estado do Rio de Janeiro, 2015, vol. 1, no. D, pp. 0–6.

[11] J. E. Soares Filho, A. de O. Feitosa, L. L. Santos, L. F. A. Campos, and R. P. S. Dutra,

“Study of the Effect of Feldspar Replacement from a Mixture of Glass / Syenite in

Technological Properties of Ceramic Coatings,” Mater. Sci. Forum, vol. 798–799, pp.

294–299, 2014.

[12] L. L. Dos Santos, J. E. Soares Filho, L. F. A. Campos, H. S. Ferreira, and R. P. S. Dutra,

“The Incorporation of the Ceramic Industry Firewood Ash into Clayey Ceramic,” Mater.

Sci. Forum, vol. 798–799, pp. 240–245, 2014.

[13] D. Gouvêa, T. Tisse Kaneko, H. Kahn, E. de Souza Conceição, and J. L. Antoniassi,

“Using bone ash as an additive in porcelain sintering,” Ceram. Int., vol. 41, no. 1, pp.

487–496, 2015.

[14] C. S. G. Penteado, E. Viviani De Carvalho, and R. C. C. Lintz, “Reusing ceramic tile

polishing waste in paving block manufacturing,” J. Clean. Prod., vol. 112, 2016.

[15] L. Carbajal, F. Rubio-Marcos, M. a. Bengochea, and J. F. Fernandez, “Properties related

phase evolution in porcelain ceramics,” J. Eur. Ceram. Soc., vol. 27, no. 13–15, pp.

4065–4069, 2007.

[16] E. El-Fadaly, “Characterization of Porcelain Stoneware Tiles Based on Solid Ceramic

Wastes,” Int. J. Sci. Res., vol. 4, no. 1, pp. 602–608, 2015.

[17] G. O. Matthew and B. O. Fatile, “Characterization of Vitrified Porcelain Tiles using

Feldspar from Three Selected Deposits in Nigeria,” vol. 3, no. 9, pp. 67–72, 2014.

[18] ABNT, Associação Brasileira de Normas Técnicas. NBR 13817 - Placas cerâmicas para

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[19] M. Dondi, G. Ercolani, G. Guarinia, C. Melandri, M. Raimondo, E. Rocha e Almendra,

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[23] ABNT - Associação Brasileira de Normas Técnicas. NBR 13818, Placas cerâmicas para

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[24] European Standard for Ceramic floor and wall tiles. Specification for dust-pressed

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ceramic tiles with a low water absorption (E ≤ 3%). Group B1., “EN 176,” 1991.

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2010.

64

3.2 ESTIMATION OF THE MINIMUM MATERIAL REMOVAL THICKNESS

DURING THE POLISHING PROCESS OF CERAMIC TILES BY LASER

TRIANGULATION

*(Published in ceramics international, https://doi.org/10.1016/j.ceramint.2017.12.032)

The possibility of controlling the thickness to be removed from each tile during the

honing/polishing process of ceramic tiles would avoid unnecessary wear of the abrasive tools,

directly minimizing energy and water consumptions. Such technology requires a conveyer belt

capable of adjusting the height of the tile surface, together with a measuring system to estimate

the most recommendable removal depth for each tile. While the former requirement is still not

promptly available in the market, the on-line characterization of the geometrical characteristics

of the tiles could be theoretically performed by many techniques. In this context, this study

presents the 3D micro-inspection by laser triangulation as a promising technique to be

employed prior to the honing process. To verify this hypothesis, six types of surfaces with

different compositions were characterized in terms of 3D topography, waviness profiles, and

surface roughness. The results indicate that the methodology adopted in this study is able to

provide precise information regarding the minimum layer to be individually removed from the

tile surfaces. In addition, it was also observed a dependence between the surface waviness is

and the tile composition. In contrast to the values typically adopted in the ceramic industries,

for the surfaces considered in this study, the minimum removal layers were found to be between

1.08% and 2.37% of the initial thickness.

Introduction

Surface inspection is an important step in the production of ceramic tiles with high

quality. However, in most cases, the surface inspection has been limited to classification cabins

in which the final product is evaluated to monitor the final quality of the product, classifying it

into the first, second or third line of quality. It is usually done visually and requires highly

skilled operators. Currently, it is possible to do this inspection by modern automatic inspection

systems which are capable of capturing images up to 160 megapixels from the surface through

CCD cameras (charge-coupled device)[1][2][3]. However, details about the evaluation criteria

of defects in final products is still limited in the literature.

https://doi.org/10.1016/j.ceramint.2017.12.032

65

In unglazed porcelain tiles, the surface inspections are only done at the end of the

finishing process, in which the final surface has a relatively high brightness. This finishing

process is commonly defined in the manufacturing units as polishing, although technically the

most appropriate designation is honing.

Usually, the honing process is composed of a sequence of 14 to 18 different abrasive

grain sizes, generally between 36 and 1500 mesh. About 10% of the tile original thickness is

removed at this stage [4][5], but there is no study justifying the need for an exact percentage of

material removal. The adoption of an exaggerated removal layer rises due to the fact the upper

surface of all the tiles onto the conveyer belt must be evenly aligned in order to avoid abrupt

breakage of the abrasive tools (fickerts). To assure such alignment, the most direct strategy is

to perform an aggressive grinding process (leveling step) right before the honing process.

Considering the entire porcelain production chain, the honing process accounts for

approximately 30-40% of all production costs [6][7], due to the high consumption of abrasive

tools, water, and energy needed to process the approximately 10% of ceramic tile plates, what

also means in a high production of waste from the wear of the ceramic plates and the fickerts.

According to Wang, Pan and Zheng [8], China, as the largest producer of ceramic coating in

the world, produces on average more than 7 millions tons of waste from such wear and tear per

year. A topographical study before the honing process may generate savings in this aspect,

avoiding unnecessary wear on the abrasive tool/ceramic pair. The major challenges, however,

are the lack of industrial devices for enabling the on-line control of the surface alignments along

the conveyer belt and for estimating the geometrical characteristics of each tile prior to the

polishing process. The present chapter is focused on the later subject.

Several studies have been done in the field of optimization of the sequence and

measurement of brightness [6][9], as well as in the optimization of the kinematics of the honing

operation as feed rate, rotational angular speed, lateral oscillation, contact pressure and honing

time [10][11][12]. How these parameters may influence the tile surface characteristics is

already reported [13]. Nevertheless, a study focused on the surface using 3D technology

remains a lack in the literature.

Laser Triangulation in porcelain stoneware tiles

The last years have been important to the development of automatic quality control

systems. The growth of inspection with 3D technologies has increased the quality control in the

66

diverse sectors of the industry. This system is capable of delivering results and maintaining a

quality of inspection on ceramic components with uneven surfaces such as porcelain tiles.

The system is designed to measure three-dimensional coordinates of points on a given

surface. The principle of measurement is triangulation. The emitting source and the CCD

camera have a fixed distance between them, here represented by Base b. The emitting and

detecting angles of the emitted beam, in relation of the base b, are represented by β and α,

respectively. The angle of reflection on the surface of the tile, ϒ, is entirely related to the

topography of the surface. Thus, depending on the variation in topography, the distance between

the sensor and the object also varies and the pulse reflected by the surface appears at different

places in the plane of the image. Thus, the topographic image of the surface under analysis is

generated. The CCD camera, the surface analysis region and laser emitter form a triangle are

shown in Figure 24, and the coordinates each point on the surface are defined by equations 1

and 2.

Figure 24: Laser triangulation in porcelain tiles with the detailed surface profile.

𝛾 = 180° − 𝛼 − 𝛽 (1)

𝑧 =𝑏 sin 𝛼 sin 𝛽

sin 𝛾 (2)

Waviness versus roughness in tile surfaces

Like metallic and polymeric surfaces, the geometry of porcelain stoneware tile surfaces is

typically described in terms of three components: form, waviness and roughness, classified

according to their wavelengths. The component form refers to oblique surfaces or lack of

parallelism, and its wavelength is virtually infinite. The ondulations usually perceived with

67

unaided eyes are commonly described as "waviness", and those of much shorter wavelengths

are known by "roughness". The roughness and waviness profiles can be determined together,

being commonly referred to as "surface texture", or separately, to identify their individual

effects on the surface of the porcelain tile.

In the technological point of view, the term polishing refers only to the elimination of

the roughness [14]. In the ceramic sector of floor tiles, however, the term "polishing"

colloquially refers to the reduction of both waviness and roughness together. The reduction of

the waviness is made in the surface leveling stage, subsequently, in the honing step, the effects

of the surface roughness are reduced, implying greater light reflectance, the glossiness. These

parameters are expressed in micrometer scale, which means that the layer to be removed to

eliminate or attenuate it will also be of the same order of magnitude, here, the layer to be

removed is defined as micrometric thickness. The majority of the thickness measurement

devices in production environments are only capable of measuring the thickness considering

the straight surface, here represented by the geometric thickness. As already mentioned, 10%

of the geometrical thickness is generally reduced in the honing process. Figure 25 illustrates

(out of scale) the surface parameters and the terms used in this study.

Figure 25: Illustration of waviness versus roughness profiles in tiles surfaces.

The main goal of this study is to use laser triangulation technique as a pre-honing

process step, in order to verify the potential use for future devices. For this purpose, different

surfaces of porcelain tiles were 3D mapped, and their waviness profiles and surface roughness

were determined.

Method

In order to develop different surfaces to be evaluated, six different compositions were

prepared. The compositions investigated include the use of two mineral residues (CK and TB),

68

as an alternative raw material since its addition may influence the microstructure and the final

properties of the tiles [15][16]. The characteristics of the raw materials used, regarding the

chemical composition and mineralogical phases, are shown in Table 6.

Table 6: Chemical and phase mineralogical characteristics of the raw materials used.

Oxides Concentration (wt.%)

Clay / Kaolin Feldspar Quartz CK TB

SiO2 56.49 / 49.21 73.61 98.03 64.99 54.96

Al2O3 24.35 / 34.78 19.37 0.89 14.98 22.51

Fe2O3 2.26 / 0.44 0.45 0.1 3.65 8.94

K2O 0.59 / 0.7 2.96 0.1 5.63 3.4

CaO 0.34 / - 0.11 - 3.34 1.57

MgO 0.69 / - - - 0.97 3.12

TiO2 0.7 / - - - 0.53 1.02

Na2O 0.17 / - 2.51 0.14 2.53 1.66

*Traces 0.31 / 0.27 0.21 0.05 0.6 0.42

**L.I 14.1 / 14.6 0.78 0.69 2.78 2.4

Phases ***Concentration (% Phases)

Kaolinite 62.00 / 97.00 - - 7.00 -

Quartz 38.00 / - 2.00 100 35.00 28.00

Albite - - - - 48.00

Ilite - - - - 24.00

Muscovite / 3.00 2.00 - 57.00 -

Rutile - - - 1.00 -

Microcline - 96.00 - - -

*Total sum of oxides with values less than 0.1%.

**Loss on ignition, 1000°C.

***Approximate values.

The developed compositions are given in Table 7, together with two main parameters

that characterize them as porcelain stoneware tile: the percentage of water absorption (WA) and

flexural strength (FS), in accordance with standards NBR 13818 [17] and EN 176 [18].

69

Table 7: Compositions of inspected porcelain tiles.

Compositions

Raw materials (wt.%) Technological Parameters

Traditional Alternatives (Post Sintering)

Clay / Kaolin Feldspar Quartz CK TB WA (%) FS (MPa)

1 30 / 15 40 10 5 - 0.09 52.11

2 30 / 15 40 5 10 - 0.18 46.56

3 30 / 15 40 - 15 - 0.09 63.45

4 20 / 20 50 5 - 5 0.18 32.59

5 20 / 20 50 - - 10 0.27 102.95

6 20 / 20 35 10 - 15 0.09 48.25

The formulations were wet-milled (1h) in a porcelain jar with alumina balls and 1% de

sodium silicate as a de-flocculant additive, obtaining 60% of solids content, and then dried at

110 °C for 24 hours. After drying, they were passed through a 35-mesh sieve. Then 8 wt.% of

water was added to assist the compaction in a hydraulic press with 45 MPa (uniaxial pressure).

For the purpose of the study, one work-piece for each composition was square molded, 45 mm

x 45 mm and after the pressing process, the work-pieces were dried at 110 °C for 24 hours

again. All the work-pieces were submitted to heat treatment in electric oven, under a heating

rate of 10 ° C / min, 20-minute sintering time at a final temperature of 1250 ° C. The cooling

was in a natural condition. These parameters aim to provide adequate conditions for obtaining

specimens with a higher rate of sintering and densification.

The processing of the raw materials and samples preparation were carried out in the

laboratory of ceramics of the Federal University of Rio Grande do Norte - UFRN, in Natal-RN,

Brazil. The 3D surface inspection was carried out at the Institute for Manufacturing Technology

and Production Systems – FBK, in the University of Kaiserslautern, Kaiserslautern, Germany.

A structured fringe projection profilometry (GFM - Mikrocad Plus) was used for this purpose.

The apparatus works based on laser triangulation (as shown briefly in section 1.1). The optical

measurement method used consists of digital strip projection with micromirror projectors. In

this method, strips with a sinus-like brightness intensity are projected onto the surface of the

measurement object with a defined triangulation angle and their image is recorded with

accuracy by CCD camera (resolution of 5 million points) and transformed into X, Y, Z

coordinates, with the support of ODSCAD software.

The region under investigation was same for the all parts evaluated, with a 1.6 mm x

1.5 mm area confined in the center of each part. A stencil has been developed to ensure the

70

marking and measurements exactly in the same region of each part. Thickness measurement

was performed with a Heidenhain SG 60M high precision thickness gauge.


For each evaluated surface three images were presented: the image obtained by the

CCD camera, a perspective view of the surface topography and the corresponding 3D color

intensity graph. These images were evaluated in order to estimate the thicknesses required to

eliminate the waviness and to reduce the roughness, separately. The results generated after the

processing of the raw images are presented in color and in micrometric scale. Fig. 26 shows an

image of the surface through a normal optical camera, indicating the area under analysis. The

geometrical thickness of each area is also presented.

Figure 26: Evaluated surfaces with the investigated area marked in red.

Figures 27 to 32 are related to compositions 1 to 4, respectively. An important feature

on the surface to be considered in the pre-abrasion inspection is the open porosity. It is possible

to see in Figure 27 that in the 3D image of composition 1 there are peaks with up to 100 μm

depth. These peaks are related to the open porosity in the surface of the sample and can be

directly seen in the optical image. Composition 1 showed a waviness variation of approximately

14 μm and a surface roughness profile ranging around 100 μm, which means that approximately

0.22% and 1.55% of the geometric thickness would need to be removed in the honing process

to promote a waviness elimination and reduction of the surface roughness of 4,58 μm,

respectively. Taking into account a scale presented on the 3D map, a total of 1.86% of the

71

sample thickness should be eliminated to attenuate the topographical variations found on this

surface.

According to many researchers [7][9][19], the first steps in the honing process of brittle

materials, such as porcelain stoneware tiles, the use of coarse abrasives result in greater material

removal and reduction of roughness. A typical threshold is the abrasive grain size #400. After

that, the surface evolution is limited to the gloss gaining. Therefore, in terms of material

removal the porcelain tile / abrasive tool interaction is more intense in the range of abrasives

between 36 and 400, inside which both the waviness and the roughness effectively decrease.

Figure 27: Micrometric characterization of the surface for composition 1.

As can be seen in Figure 28, the composition 2 has a higher concentration of open and

deeper pores on the surface, noticeably greater than observed on the surface of composition 1.

This directly reflects the water absorption values exhibited in Table 1. Due to this, the surface

presented an average roughness of 5,79 μm, requiring a reduction of 2,37% of the initial

thickness to attenuate the ripple and roughness effects.

72


The composition 3 (Figure 29) showed similar characteristics from compositions 1 and

2, such as presence of deeper regions relate a porosity of up to 100 μm. This was perfectly

coherent since the raw material processing parameters were invariable. in these first three

compositions, and they all have the same composition base: materials that provide plasticity

and fluxes maintained in the same amount in the formulations.

With a surface roughness of 5.80 μm and a topographic variation of approximately 140

μm, a decrease of 2.02% of thickness initially measured for the delineated area will required.

The waviness profiles in the first three compositions evaluated were maintained between 14

and 16 μm.

73


The base (amount of traditional raw materials) for compositions 1, 2 and 3 differs from

the base of compositions 4, 5 and 6, furthermore, the compositions of the same base differ from

one another by the amount of alternative materials incorporated. The topographic mappings of

the last three compositions studied can be seen in Figures 30, 31 and 32.

Composition 4 (Fig. 30) revealed relatively deep peaks and was not evidenced in

compositions 5 (Fig. 31) and 6 (Fig. 32), both of which presented lower surface roughness

profiles. To reduce the effects of these two important features in compositions 4 and 5, removals

of 2.37% and 1.08%, respectively, are required. The composition 5 was one that had the least

topographical variations.

74



Among the compositions evaluated, the composition 6 (Fig. 32) presented the greatest

level of waviness, as shown in 3D topography. It requires about 0.62% removal from the initial

thickness to mitigate this effect. On the other hand, deep porosity was not observed, in contrast

to the others compositions also evaluated in this study. With the addition of the average surface

75

roughness of 4.93 μm, a total removal of 1.59% in thickness could be recommended for this

composition.


Relevant defects such as cracks and micro cracks were not detected in the analyzed

regions for all surfaces. Considering the waviness and roughness effects together, the maximum

thickness removal was required for composition 2 (2.37%) and the minimum by composition 5

(1.08%). Even considering an extra margin of material removal, a total removal of about 10%

geometric thickness, usually adopted appears to be excessive, leading to unnecessary wear in

the set porcelain tile/abrasive tool.

It must be remembered that the surfaces have been inspected before the honing process.

The access to such information offers the possibility of analyzing the effects of the sequence of

abrasive blocks on the tile surface, regarding material removal and the glossiness. Such analysis

must be carried out for each type of formulation with different proportions of raw materials.

Conclusions

Based on the results obtained, the following conclusions can be drawn:

- For all compositions tested, the 3D inspection used in this study as a pre polishing

process stage is capable to provide important information about the topography of the tile

surface in the micro scale, such as waviness profile and surface roughness. This can be of

extreme importance in helping decision-making regarding the entire production process, from

76

the selection of raw materials and optimization of compositions, to the leveling and honing

itself, by optimizing the sequence of abrasive grains to be used.

- Since the depths necessary to level the surface and to do the polishing could be known

The 3D inspection adopted together with a suitable choice of kinematics for the honing process

would be capable of save energy, water and abrasive tool, which in turn would lead to

significantly reduction in the production costs. In this way, there might be an optimization in

the wear evolution of both porcelain tile and the abrasive tool.

- More detailed topographic mapping studies could still be performed to evaluate the

waviness profile in different regions of larger ceramic plates, as well as in different samples

collected from different regions inside the kilns during production. Thus, by tracking the tiles

individually, an overview can be constructed about the expected levels of waviness and

roughness, taking into account the temperature gradients.

Finally, the 3D inspection seems to be a very useful tool to characterize and follow the

surface evolution of different porcelain stoneware tile compositions.

References

[1] M.L. Bueno, M.R. Stemmer, P.S.D.S. Borges, Inspeção Visual Automática de Peças

Cerâmicas via Inteligência Artificial, Cerâmica Ind. 5 (2000) 29–37.

[2] H. Elbehiery, A. Hefnawy, M. Elewa, Surface Defects Detection for Ceramic Tiles Using

Image Processing and Morphological Techniques, Proc. World Acad. Sci. Eng. Technol.

Vol 5. 5 (2005) 158–162.

[3] R. Mishra, C.L. Chandrakar, Surface defects detection for ceramic tiles using image, Int. J.

Data Warehous. 4 (2012) 51–58.

[4] W.S. Wiggers, R. a Santos, D. Hotza, Evolução da Superfície do Porcelanato ao Longo do

Processo de Polimento, Cerâmica. 12 (2007) 27–30.

[5] E. Sánchez, M.J. Ibáñez, M.J. Orts, V. Cantavella, Polishing porcelain tile. Part 1: wear

mechanism, Am. Ceram. Soc. (2002) 50–54.

[6] I.M. Hutchings, Y. Xu, E. Sánchez, M.J.I.M.F. Quereda, Optimización del proceso de

pulido para piezas de gres porcelánico, in: Qualicer, 2006: pp. 405–414.

[7] I.M. Hutchings, K. Adachi, Y. Xu, E. Sánchez, M.J. Ibáñez, M.F. Quereda, Analysis and

laboratory simulation of an industrial polishing process for porcelain ceramic tiles, J. Eur.

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Ceram. Soc. 25 (2005) 3151–3156. doi:10.1016/j.jeurceramsoc.2004.07.005.

[8] S. Ke, Y. Wang, Z. Pan, C. Ning, S. Zheng, Recycling of polished tile waste as a main raw

material in porcelain tiles, J. Clean. Prod. 115 (2016) 238–244.

doi:10.1016/j.jclepro.2015.12.064.

[9] F.J.P. Sousa, N. Vicente, W.L. Weingaertner, O.E. Alarcon, Glossiness distribution over

the surface of stoneware floor tiles due to the polishing process, J. Mater. Sci. 42 (2007)

10124–10132. doi:10.1007/s10853-007-2118-4.

[10] F.J.P. Sousa, J.C. Aurich, W.L. Weingaertner, O.E. Alarcon, Optimization of the

Kinematics Available in the Polishing Process of Ceramic Tiles by Computational

Simulations, J. Am. Ceram. Soc. 92 (2009) 41–48. doi:10.1111/j.1551-2916.2008.02850.x.

[11] A.S.B.D.S. Nascimento, F.J.P. Sousa, Distribution of contact pressure over the surface of

ceramic floor tiles during the polishing process, J. Eur. Ceram. Soc. 34 (2014) 3209–3215.

doi:10.1016/j.jeurceramsoc.2014.04.032.

[12] F.J.P. Sousa, J.C. Aurich, W.L. Weingaertner, O.E. Alarcon, Analytical Determination of

the Distribution of Polishing Time over the Surface of Polished Tiles, J. Am. Ceram. Soc.

90 (2007) 3468–3477. doi:10.1111/j.1551-2916.2007.01956.x.

[13] C.Y. Wang, T.C. Kuang, Z. Qin, X. Wei, How Abrasive Machining Affects Surface

Characteristics of Vitreous Cermamic Tile, Am. Ceram. Soc. (2003) 9201–9208.

[14] F.J.P. Sousa, L. L, R. G, Polishing, in: CIRP Encycl. Prod. Eng., Springer-Verlag Berlin

Heidelberg, 2014: pp. 957–962. doi:10.1007/978-3-642-20617-7.

[15] ABNT - Associação Brasileira de Normas Técnicas. NBR 13818, Placas cerâmicas para

revestimento - Especificação e métodos de ensaios, Brasil, 1997.

[16] European Standard for Ceramic floor and wall tiles. Specification for dust-pressed ceramic

tiles with a low water absorption (E ≤ 3%). Group B1., EN 176, 1991.

[17] J.E. Soares Filho, A. de O. Feitosa, L.L. Santos, L.F.A. Campos, R.P.S. Dutra, Study of the

Effect of Feldspar Replacement from a Mixture of Glass / Syenite in Technological

Properties of Ceramic Coatings, Mater. Sci. Forum. 798–799 (2014) 294–299.

doi:10.4028/www.scientific.net/MSF.798-799.294.

[18] L.L. Dos Santos, J.E. Soares Filho, L.F.A. Campos, H.S. Ferreira, R.P.S. Dutra, The

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Incorporation of the Ceramic Industry Firewood Ash into Clayey Ceramic, Mater. Sci.

Forum. 798–799 (2014) 240–245. doi:10.4028/www.scientific.net/MSF.798-799.240.

[19] I.M. Hutchings, Y. Xu, E. Sánchez, M.J. Ibáñez, M.F. Quereda, Development of surface

finish during the polishing of porcelain ceramic tiles, J. Mater. Sci. 40 (2005) 37–42.

79

3.3 POLISHING PERFORMANCE OF GREEN CERAMIC TILES MADE

WITH WASTES FROM BRICKS AND ROOF TILES

*(Submitted to the Journal of Europe Ceramic Society)

The industrial production of bricks and roof tiles plays a significant role in the

economy of the state of Rio Grande do Norte, in northeastern Brazil. In the last years, the

production has grown exponentially, and so has the production of the waste from the

breakage of those materials, known as grog of red ceramics. As consequence, the concern

has also arisen over the final disposal and reuse of this waste. Several studies reported the

incorporation of this residue in the ceramic matrix for porcelain stoneware tile, but only

focused on the evaluation of basic technological properties. The present work investigates

the influence of waste over the gloss-gaining curve of the final products. The polishing

behaviour of three different green compositions, with 5, 10 and 15% of grog of red ceramic

incorporated, was evaluated in comparison with a standard composition, waste-free. For this

purpose, porcelain stoneware tile samples were prepared and their corresponding

microstructures were characterized. The industrial polishing condition was reproduced in

laboratory scale, keeping the same abrasive agent, SiC, in an automatic metallographic

polishing machine. The roughness and gloss performances through the abrasive sequence

adopted were investigated. Additionally, the texture of the surfaces was also evaluated. The

results indicate the feasibility and the promising potential of the polishing process of

formulations containing recycled grog. The gloss levels were compatible with commercial

polished porcelain stoneware tiles currently available at the market.

Introduction

Porcelain stoneware tile is considered as the highest-performance product between

the large range of ceramic tiles. In addition, it offers a polished version with great aesthetic

impact, very well accepted in the market,. However, a strict control in all production stages

is required to ensure homogeneity in terms of color tonality and surface finish, since no

further coatings like ceramic glaze is adopted.

The industrial polishing process, from the technological point of view, consists of

elimination or severe reduction of roughness [1], causing the glossiness of the polished

surface to to increase. The behavior that describes the development of the roughness

80

reduction and the gloss enhancement in porcelain stoneware tiles was evaluated and

presented with a pioneering approach by Hutchings et al [2].

High levels of gloss, approximately 80 units of gloss, are achieved after the porcelain

stoneware tiles pass through a gradual sequence of 30 to 32 tangential tools, colloquially

known as polishing heads, positioned in series and with decreasing sizes of abrasives

particles, constituting a polishing line [2].

Currently, most of porcelain tile factories uses two different abrasives particles:

silicon carbide (SiC) and synthetic diamond. The diamond particles are dispersed in a

metallic matrix and SiC particles in a cement matrix, usually magnesian (magnesium

oxychloride cement), resulting in abrasives blocks, known as fickerts. Figure 33 shows the

two types of fickerts mostly used by the industries.

Figure 33: a) Cement fickert (new) and b) diamond fickert (used). Author.

Figure 34 presents a typical industrial polishing head with six attached fickerts. The

swinging motion performed by those fickerts is explained in the inset.

81

The literature is able to offer many fruitful results about the polishing process of

porcelain stoneware tiles producted with usual raw materials. Most of those investigations

are focused on optimizations of production parameters, such as kinematics [3], abrasive

sequence [4][5], surface inspection [6], and others phenomenological aspects of the

polishing process of commercial compositions of porcelain stoneware tiles [7][8], as well as

the examination of the polishing final quality with the consumer view [9].

There are also several studies dealing with recycling and reuse of industrial wastes

as alternative raw materials in porcelain tile manufacturing [10][11][12], but they focuse on

the ceramic processing and evaluation of the resulting mechanical properties

[13][14][10][15].

Investigations considering the two aforementioned approches still remain a lack in

literature, despite the potential interest from the economic and environmental point of view.

In this context, the present chapter addresses the polishing process of green porcelain

stoneware tiles produced with an alternative raw material: the waste from shards resulting

from the breakage of red ceramic bodies, mainly roof tiles and bricks (TB), colloquially

known as "grog" of red ceramics. This waste is quite abundant in northeastern Brazil,

especially in the state of Rio Grande do Norte.

The polishing process followed a specific kinematics. The roughness and gloss of

polished porcelain tiles were analyzed after each phase of the process. Together with the

usual technological properties, like water absorption and flexural strength, the level of

surface quality was compared with the standard composition and also with the main results

found in the literature, in order to evaluate if the introduction of the waste is feasible from

Figure 34: Representation of the fickerts mounted to the polishing head. Detail of the

swing motion performed by the fickert. Author.

82

the perspective of the polishing process. Additionally, the surface texture evolution through

the polishing process was also investigated.

Experimental

Manufacturing process

The workpieces of porcelain stoneware floor tiles used for the polishing testing were

obtained with TB incorporated in 5, 10 and 15 wt.% and the standard mixture, waste-free.

The formulations were wet-milled (1 hour) aiming to obtain a liquid paste with 60% of solids

content and then dried at 110 °C for 24 hours. Then 8 wt.% of water was added to assist the

compaction in a hydraulic press with 45 MPa (uniaxial pressure) to get the final shape (45x45

mm). The workpieces were dried at 110 °C for 24 hours and submitted to heat treatment in

electric oven, under a heating rate of 10 ° C / min, 20-minutes sintering time at a final

temperature of 1250 ° C with natural cooling condition.

The main technological properties used to classify the high-performance tiles as

porcelain stoneware tiles are the percentage of water absorption (WA) and mechanical

performance by flexural strength (FS). In the current experiment, both properties were

properly characterized in accordance with NBR 13818 [16] and EN 176 [17] standards. In

addition, not less important, the others technological properties were also measured after the

heat treatment. All the technological parameters and the established composition are shown

in Table 8.

Table 8: Compositions adopted and their resulting technological properties.

(wt.%) Raw Compositions

Aft

er s

inte

ring Technological Compositions

Materiais STD C5 C10 C15 properties STD C5 C10 C15

TB - 5 10 15 Water absortion (%) 0.18 0.18 0.27 0.09

Feldspar 50 50 50 35 Flexural strength (MPa) 42.6 32.59 102.95 48,45

Quartz 10 5 - 10 Apparent porosity (%) 0.42 0.45 0.64 0.21

Clay/Kaolin 40 40 40 40 Linear shrinkage (%) 7.21 10.00 9.58 8.51

The ceramic processing of the raw materials for samples preparation were carried out

in the laboratory of ceramics of the Federal University of Rio Grande do Norte - UFRN, in

Natal-RN, Brazil.

83

Polishing

Three workpieces for each composition were polished using an automatic

metallographic polishing machine mecatech 334 from Presi. An adaptation was necessary

to fit the square samples in the round sample holder, then, a round resin base was fixed on

the back side of the each workpiece. SiC emery papers were used to promote the abrasive

contacts, The sequence of the abrasive particles sizes used is described as the sequence of

the grit numbers: #40, #60, #80, #120, #180, #240, #320, #400, #600, #1200, and #2500.

This sequence of abrasive size is similar to that practiced in the industrial process. The

equipment and the polishing system are shown in figure 35.

Figure 35: (a) Polishing machine adopted. (b) Detail of the motion sources (schematic

plan view)(c) Detail of the loading system (schematic side view).

Aiming to evaluate whether the difference in the microstructure caused by the

insertion of such waste, as well as its quantity variation in the compositions affects the

glossiness gain behavior, the kinematic parameters were kept constant. The kinematic

parameters adopted can be found in table 9.

Table 9: Kinematics parameters adopted.

Normal load 30 N

Rotations Workpieces (ω1) Counterclockwise 150 RPM

Emery paper (ω2) Clockwise 600 RPM

84

The kinematic parameters were in accordance with fruitful investigations available

in the literature using a similar system [2][4].

In order to reproduce an industrial polishing line with three polishing heads for each

abrasive size as close as possible, the workpieces were polished three times (180 seconds

each time) for each grit number of the abrasive sequence adopted.

It is well known that the polishing time is also an important variable in the process

and directly affects the final gloss. Thus, the polishing time was extrapolated to a saturated

condition and kept invariable to ensure that the microstructure was the only variable

throughout the polishing process.

Measurements

The surface roughness and the gloss pattern were measured between each step of the

polishing process. The workpieces´ surface were equally divided into 5 regions, one central

(c) and other four into the peripherical regions. One measurement for each direction at the

center of the region was taken for both parameters already mentioned, as can be seen in Fig.

36. Additionally, the material removal is also considered and analytically determined in the

investigation.

A glossmeter (ZGM 1120 - Zehntner) was used to take the gloss measurements. The

incident angle was θ = 60°, and the results were expressed in gloss units, (GU). For

roughness measurements, one roughness meter (MarSurf M 400 - Mahr) was used.


measurements.

The microstructure of the fired compositions mentioned was analyzed by Quanta

600F scanning electron microscope (SEM) equipped with an energy dispersive X-ray

85

attachment (EDX). The fractured surface observation was carried out after chemical etching

with a hydrofluoric acid solution for 3 minutes (HF - 2 vol %).

The evolution of the surface finish during polishing was investigated by a confocal

3D microscope (Nano Focus, model µsurf Explorer). The system works based on non-

contact surface metrology with high-resolution sensors and linear encoders on X, Y and Z

axes. The investigated region was aways kept confined to an elliptic area of 1.5 mm x 1.5

mm located at the center of each work-piece, as indicated by C in Fig. 36. A stencil was

developed to ensure the marking and measurements exactly in the same region of each part.


Microstructure

A secondary electron SEM image was performed for all compositions and are shown

in Fig. 37 for the standard composition and in Fig. 38 for 3 green compositions [a) 5%, b)

10% and c) 15%]. Firstly, the presence of the primary and secondary mullite crystals (needle

shape) could be observed embedded in the interfaces between the quartz crystals and the

glassy matrix in all compositions. The central region of each SEM image was chosen to have

a qualitative analysis by EDX.

Figure 37: Microstructure, as seen by SEM, of the composition free of waste.

For the compositions C5 and C10, the qualitative analysis showed the presence of

Al, Si in higher proportions, and Na, K, Ca, and Mg in smaller proportions for the regions

86

of the positions 1 and 2 and practically only Si for the spot 3. Still in Fig. 38, all the positions

measured for the composition C15 has shown majority Al and Si.

Figure 38: Microstructure (as seen by SEM) of the compositions C5 – (a), C10 – (b)

and C15 – (c) with details of EDX analysis for three different positions each one.

The mullite regions in all 3 eco-friendly ceramic tiles were enriched with iron from

the used waste. Probably, the iron (in Fe+3 ions) is bound in the mullite structure interstices.

Gloss and roughness

Figure 39 exposes the experimental average results for the polishing of the standard

composition, free of waste. Fig. 40 and Fig. 41 shows the gloss and roughness experimental

average results from the laboratory polishing process, respectively. The curves show the

evolution of roughness and gloss gain through the abrasive grit sequ ence adopted. The

experimental points within the range between 0 (natural surface) and 500 on the abrasive

grit axis represents the grits #40, #60, #80, #120, #180, #240, #320 and #400, in this

sequence.

87

Figure 39: Gloss (a) and roughness (b) development through the abrasive sequence for

the stardard composition.

Comparing the maximum glossiness values obtained for the standard composition

(59.0 GU) and those of the other three compositions studied (C5 - 70.6 GU, C10 - 72.7 GU

and C15 - 65.8 GU) it is possible to verify that the introduction of the grog favors the final

glossiness in all three incorporations, being the composition C10 that presented the best final

gloss. This improvement of the final gloss can be attributed to the probable enrichment of

the mullite regions with the additional iron coming from the waste used.

The gloss average values found for the studied compositions are similar to some

found in the literature. Hutchings et al. [5], using a similar polishing system, obtained levels

of 70 GU. Wiggers et al. [18] have used an industrial porcelain tile and an industrial-like

polisher, but without the transverse movement of the polishing heads, obtained a maximum

level of approximately 75 GU.

88

Figure 40: Surface gloss development through the abrasive sequence for the

compositions with waste incorporated.

Figure 41: Surface roughness development through the abrasive sequence for the

compositions with waste incorporated.

89

As can be seen in Fig. 40c and Fig. 41c, representing polishing evolution of the

composition C15, the surface gloss and roughness evolution in grits #240 and #320 become

worse in relation to the previous grits. This may be attributed to the opening of some

previously closed pores or by the chemical mechanism, where the removal of confined

material occurs to a chemically modified layer. In the following grits, the pores become

smaller and also mitigated by the mechanism of surface flow, in which a resettlement of

material by plastic deformation occurs [1].

Despite the small difference in the roughness values at the end of the process, the

three curves showed similar behavior. In addition, it is clear that the roughness reduction

rate is bigger between the beginning of the process and the grit #400, after that, the roughness

reduction rate is smoothed. That behavior is characterized by the changing of material

removal mode, from brittle to ductile removal [19][20].

Morphologic evaluation

Once the polishing process started the morphological variations suffered by the

surfaces after the polishing step with #600 grit and polished after #2500 were checked and

are presented in Figures 42, 43 and 44.

Figure 42: Surface evolution of composition C5.

Like expected, there is a great difference in surface morphology between the samples

under natural conditions and after #600. On the other hand, no significant differences were

observed in texture between #600 and #2500. The surface textural evolution for all

compositions confirms the smoothness of the polished surface.

90


Still regarding morphology, it is possible to verify the presence of pores with very

irregular shapes. During the whole polishing process some pores were completely removed,

whereas others that were closed before, become opened.


According to the measurements carried out within the methodology shown in Fig.

36, it was verified that the surface gloss distribution of all compositions are not

homogeneous. Nevertheless, this heterogeneous distribution of the gloss pattern in porcelain

stoneware tiles has already been reported by Sousa et al [21] and the possible explanation

for this phenomenon is associated the choice of kinematics to be adopted and it is not an

intrinsic characteristic of the porcelain stoneware tile composition.

Given the reasoning above, it can be observed that in laboratory conditions, the

addition of 5, 10, 15 wt.% grog of red ceramic did a positive change the final gloss

characteristics when compared to the glossiness standard obtained after the polishing of

waste-free composition.

In addition, it was observed that the sequence of 12 different abrasive grains,

simulating a sequence of 36 polishing heads, was able to offer good final gloss levels, up to

72,7 GU, as occurred for the composition C10%. The abrasive sequence used may be

optimized to achieve even higher gloss levels than those obtained herein.

91

Conclusions

The experimental results presented in this chapter point out that innovative green

products with excellent performance combined with great aesthetic can be produced by the

incorporation of TB waste into porcelain stoneware tiles compositions. This alternative not

only saves original raw materials, but also adds value to an industrial waste that is currently

being used simply as landfill in civil construction or otherwise wrongly disposed in the

environment.

In order to obtain higher values of gloss, further additions of TB waste may be still

possible, and a limit may be reached, above which the polishing is no longer of interest from

the aesthetic point of view.

New industrial wastes should be tested aiming to obtain different microstructures to

be polished in order to achieve the gloss pattern for each type of waste, after the adoption of

the following technical topics:

The use of the same type of abrasives used in industry (SiC or diamond fickerts).

Specific machinery for the polishing of porcelain tiles, with transverse motion.

Optimized kinematics and polishing time.

References

[1] F. J. P. Sousa, L. L, and R. G, “Polishing,” in CIRP Encyclopedia of Production

Engineering, Springer-Verlag Berlin Heidelberg, 2014, pp. 957–962.

[2] I. M. Hutchings, Y. Xu, E. Sánchez, M. J. Ibáñez, and M. F. Quereda, “Development

of surface finish during the polishing of porcelain ceramic tiles,” J. Mater. Sci., vol.

40, pp. 37–42, 2005.

[3] F. J. P. Sousa, D. S. Hosse, J. C. Aurich, M. Engels, W. L. Weingaertner, and E.

Alarcón, “Simulation and analysis of an alternative kinematics for improving the

polishing uniformity over the surface of polished tiles,” Boletín la Soc. Española

Cerámica y Vidr., vol. 49, no. 4, pp. 242–247, 2010.

[4] I. M. Hutchings, K. Adachi, Y. Xu, E. Sánchez, M. J. Ibáñez, and M. F. Quereda,

“Analysis and laboratory simulation of an industrial polishing process for porcelain

92

ceramic tiles,” J. Eur. Ceram. Soc., vol. 25, no. 13, pp. 3151–3156, 2005.

[5] I. M. Hutchings, Y. Xu, E. Sánchez, and M. J. I. M. F. Quereda, “Optimización del

proceso de pulido para piezas de gres porcelánico,” in Qualicer, 2006, no. 1, pp. 405–

414.

[6] J. E. Soares Filho, J. C. Aurich, F. J. P. Sousa, R. M. Nascimento, and C. A.

Paskocimas, “Estimation of the minimum material removal thickness during the

polishing process of ceramic tiles by laser triangulation,” Ceram. Int., no. July, pp. 0–

1, 2017.

[7] F. J. Pinheiro Sousa, J. Seewig, C. Chiamulera, O. E. Alarcon, and W. L. Weingärtner,

“Evolution of Wear on the Abrasive Tool during the Polishing of Porcelain Tile Using

Morphological Space,” Adv. Mater. Res., vol. 906, pp. 293–302, 2014.

[8] A. S. B. D. S. Nascimento and F. J. P. Sousa, “Distribution of contact pressure over

the surface of ceramic floor tiles during the polishing process,” J. Eur. Ceram. Soc.,

vol. 34, no. 13, pp. 3209–3215, 2014.

[9] C. Cass and B. E. D. T. Técnico, “Optical haze on polished porcelain tiles, a consumer

’ s perspective,” in Qualicer 2010., 2010, pp. 1–16.

[10] R. M. Novais, M. P. Seabra, and J. A. Labrincha, “Wood waste incorporation for

lightweight porcelain stoneware tiles with tailored thermal conductivity,” J. Clean.

Prod., vol. 90, pp. 66–72, 2015.

[11] M. F. Serra, M. S. Conconi, G. Suarez, E. F. Aglietti, and N. M. Rendtorff, “Volcanic

ash as flux in clay based triaxial ceramic materials, effect of the firing temperature in

phases and mechanical properties,” Ceram. Int., vol. 41, no. 5, pp. 6169–6177, 2015.

[12] M. Awaad, S. M. Naga, and N. El-Mehalawy, “Effect of replacing weathered feldspar

for potash feldspar in the production of stoneware tiles containing fish bone ash,”

Ceram. Int., vol. 41, no. 6, pp. 7816–7822, 2015.

[13] K. Kim, K. Kim, and J. Hwang, “Characterization of ceramic tiles containing LCD

waste glass,” Ceram. Int., vol. 42, no. 6, pp. 7626–7631, 2015.

[14] D. Gouvêa, T. T. Kaneko, H. Kahn, E. De Souza Conceição, and J. L. Antoniassi,

“Using bone ash as an additive in porcelain sintering,” Ceram. Int., vol. 41, no. 1, pp.

487–496, 2015.


industrial wastes in ceramic manufacturing: State of art and glass case studies,”

Ceram. Int., vol. 42, no. 12, pp. 13333–13338, 2016.

93

[16] ABNT, Associação Brasileira de Normas Técnicas. NBR 13817 - Placas cerâmicas

para revestimento - Classificação. Brasil, 1997, pp. 1–3.

[17] European Standard for Ceramic floor and wall tiles. Specification for dust-pressed

ceramic tiles with a low water absorption (E ≤ 3%). Group B1., “EN 176,” 1991.

[18] W. S. Wiggers, R. A. Santos, and D. Hotza, “Evolução da Superfície do Porcelanato

ao Longo do Processo de Polimento,” Cerâmica, vol. 12, pp. 27–30, 2007.

[19] A. Olenburg, J. C. Aurich, F. J. P. Sousa, “Polishing process of ceramic tiles –

influence of tool wear on gloss,” in Proceedings of Qualicer: World congress on

ceramic tile quality Vol. I. Castellón: Cámara Oficial de Comercio, Industria y

Navegación, 2014, no. 1, pp. 1–13.

[20] E. Sánchez, M. J. Ibáñez, M. J. Orts, and V. Cantavella, “Polishing porcelain tile. Part

1: wear mechanism,” Am. Ceram. Soc., no. September, pp. 50–54, 2002.

[21] F. J. P. Sousa, N. Vicente, W. L. Weingaertner, and O. E. Alarcon, “Glossiness

distribution over the surface of stoneware floor tiles due to the polishing process,” J.

Mater. Sci., vol. 42, no. 24, pp. 10124–10132, 2007.

94

3.4 EVALUATION OF SURFACE QUALITY AFTER POLISHING OF ECO-

FRIENDLY PORCELAIN STONEWARE TILES MADE WITH KAOLIN ORE

WASTE

*(Submitted to the Journal of the American Ceramic Society)

To effectively utilize the waste from kaolin ore in ceramic floor tiles, three compositions

with 10, 15 and 20% of additions were successfully developed, based on the excellent

technological properties reached. A standard composition (waste-free) was also developed for

comparison purposes. Beyond the usual technogical properties, this research investigates the

influence of the addition of the waste from kaolin ore on the final surface glossiness via polishing

process, including microstructural and morphological evaluation, still not addressed by previous

literature. The results showed that extra mullite source from the kaolin ore waste may influence

positively the final quality of the porcelain stoneware tiles. The highest glossiness reached was 70

gloss units for the mixture C20, which represented an increase of 18.6% in the final surface

glossiness in comparison with the standard composition. This study therefore reveals that kaolin

ore wastes can be used to produce eco-friendly porcelain stoneware tiles in the polished version,

without damaging the final level of gloss.

Introdution

Nowadays one of the biggest concerns of humanity is the conservation of nature, a practical

challenge is the development of new routes for the sustainable development and management of

industrial waste. Motivated by the reduction in the consumption of natural raw materials, which

are increasingly scarce, and by the need to add value to increasingly abundant industrial wastes,

this investigation encompasses a sequence of investigations aiming to produce environmental eco-

friendly polished porcelain stoneware tiles [1]. Specifically, the present study investigates the

feasibility of using the first waste generated from the kaolin ore beneficiation process, colloquially

known as coarse kaolin (CK), as a partial replacement in the production of eco-friendly porcelain

stoneware tiles, including the polishing process.

In Brazil, the state of Rio Grande do Norte is one of the largest producers of kaolin ore,

consequently it is one of the largest generators of the waste from the processing of this material.

Currently, the introduction of the waste from kaolin beneficiation process in formulations

of various mullite-based ceramic products is very well-known in the literature [2][3][4]. Regarding

porcelain stoneware tile production, the use of industrial wastes in formulations not only saves

natural raw material [5][6][7], but also saves energy. For instance, Sangsom Chitwaree et al [8]

95

reported a saving of up to 30% in the energy consumption of the production. However, when it

comes to the polishing process and the resulting level of gloss, the behavior of the compositions

conceived with wastes remains a lack in literature.

The industrial process for polishing porcelain stoneware tiles involves dozens of tangential

polishing tools, known as polishing heads, each one requiring six abrasive blocks (fickerts),

symmetrically arranged. There are two types of fickerts used in the industrial polishing process,

one composed of abrasive particles of silicon carbide (SiC), and another of synthetic diamond,

usually 10 wt.% [9]. The diamond particles are dispersed in a metallic matrix whereas SiC fickerts

in a cement matrix, usually magnesian (magnesium oxychloride cement). In both cases, the surface

of the abrasive fickerts contacting the surface to be polished exhibits a certain convexity, so that

only a strip of the block has an effective contact zone with the tile surface the during polishing

operation.

One of the most important factors in the polishing operation is the definition of the abrasive

sequence. According to Wang et al [10], the polishing operation can be subdivided into three sub-

steps: grinding (between abrasives grits #36 – #280), semi-polishing (#320 - #600) and polishing

(#800 – Lux).

Some hypothesis about the interaction between the abrasive particles and the surface

during polishing were listed by Sousa et al [11] and shown in figure 45.

Figure 45: Interactions between the abrasives particles and the porcelain stoneware tiles

surface. Author.

Figure 45 illustrates the wear hypothesis, occurring the generation of very fine chips, as a

consequence of cutting interactions. Figure 1d refers to surface flow hypothesis that addresses the

heat generated by the friction can reach hundreds of degrees Celsius and this can cause the

96

softening of the material, which facilitates the resettlement to the lower regions, like porous. Figure

1e highlights the occurrence of chemical reactions during the polishing process.

Previous fruitful studies have investigated the mechanisms of the polishing process of

commercial porcelain stoneware tiles with emphasis on the influence of the kinematics, abrasive

tools and others variables [10][12][13][14], emphasizing the need for investigations of polishing

of eco-friendly compositions. Thus, this study deals with the polishing process of eco-friendly

porcelain stoneware tiles produced with the waste from kaolin ore as an alternative raw material.

Experimental

Formulation

A standard (STD), composition free of waste, and three different eco-friendly porcelain

stoneware tiles compositions were conceived with the addictions of 10, 15 and 20% of the first

waste generated from the kaolin beneficiation process, colloquially known as coarse kaolin (CK),

as can be seen qualitatively in Table 10.

Table 10: Compositions adopted and their resulting technological properties.

(%) Raw Compositions

Aft

er s

inte

rin

g Technological Compositions

Materiais STD 10% 15% 20% properties STD 10% 15% 20%

CK - 10 15 20 Water absortion (%) 0.18 0.18 0.09 0.09

Feldspar 50 40 40 39 Flexural strength (MPa) 42.6 46.56 63.45 47,81

Quartz 10 5 - - Apparent porosity (%) 0.42 0.44 0.22 0.22

Clay/Kaolin 40 45 45 41 Linear shrinkage (%) 7.21 10.17 10.40 10.19

The mixtures were developed so that they are safely within the porcelain stoneware tile

region in the triaxial system for ceramic products, shown in Figure 46.

97

Figure 46: Triaxial system (wt.%) for ceramic products with the mixtures within the porcelain

stoneware tile region, with detail of the interested region.

In the preliminary mineralogical investigation, the residue used has shown the quartz and

Muscovite phases in higher intensity and kaolinite in minor, as well as rutile traces. The traditional

raw materials used in the standard formulation, such as clay, quartz, feldspar, and kaolin, have

been characterized in other previous studies [15][16].

The mixtures were wet-milled for 1h aiming to obtain a liquid paste with 60% of solid

content and then dried at 110 °C for 24h. Then 8 wt.% water was added to assist the compaction

in a hydraulic press with 45 MPa (uniaxial pressure) to get the final square shape samples (45x45

mm). The workpieces were dried at 110 °C for 24h and submitted to heat treatment in electric

oven, under a heating rate of 10 ° C / min, 20min sintering time at a final temperature of 1250 °C

with a natural cooling condition.

Polishing

In order to reproduce an industrial polishing condition as close as possible, an automatic

laboratory polishing machine (mecatech 334, Presi) was used. However, an adaptation was

necessary to fit the square samples in the round sample holder, in whicha round polymeric base

was fixed on the back side of the each workpiece. The equipment and the polishing system are

shown in Figure 47.

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Figure 47: (a) Polishing machine adopted. (b) Detail of the system mounted. (c) Detail of the

adaptation (schematic side view).

SiC emery papers were used to promote the abrasive contacts, the sequence of the abrasive

grit numbers and it's average particles sizes (A.S.) used are described in Table 11, related to the

respective sub-steps of the polishing process.

Table 11:Abrasive sequence adopted.

Three workpieces for each one the the four abovementioned composition were polished

and subjeted to two subsequential analyses. The first analysis was devised to check the effect of

the introduction of Coarse Kaolin on the resulting microstrure, whereas the second analysis

evaluates the influence of the microstruture on the gloss-gaining behavior under polishing. The

kinematic parameters were kept constant and can be found in Table 12.

Table 12: Kinematics parameters adopted.

The kinematic parameters were in accordance with fruitful investigations available in the

literature using a similar system [9][17].

G

rindin

g

Grit A.S. (µm)

S

emi-

poli

shin

g

Grit A.S (µm)

Poli

shin

g

Grit A.S (µm)

#40 390.0 #320 29.2

#1200 3 #60 270.0

#80 190.0 #400 17,3

#120 110.0

*Lux <3,0 #180 75.0 #600 9,3

#240 63.0 (#2500)

Normal load 30 N

Rotations Workpieces (Counterclockwise) 150 RPM

Emery paper (Clockwise) 600 RPM

99

In order to reproduce an industrial polishing line with three polishing heads for each

abrasive size as close as possible, the workpieces were polished three times (180 seconds each

time) for each grit number of the abrasive sequence adopted.

It is well known that the polishing time is also an important variable in the process and

directly affects the final gloss, due to this, the polishing time was extrapolated to a saturated

condition and kept invariable to ensure that the microstructure was the only variable throughout

the polishing process.

The surface roughness and the gloss pattern were measured between each step of the

polishing process. The workpieces´ surface were equally divided into 5 regions, one central (c)

and other four into the peripherical regions. One measurement for each direction at the center of

the region was taken for both parameters already mentioned, as can be seen in Fig. 48.

A glossmeter (model ZGM 1120 – Zehntner) was used to take the gloss measurements.

The incident angle was θ = 60°, and the results were expressed in gloss units, (GU). For roughness

measurements (Ra in µm), one roughness meter MarSurf M 400 from Mahr was used.


measurements.

The microstructure of the fired compositions mentioned was analyzed by (Quanta 600F)

scanning electron microscope (SEM) equipped with an energy dispersive X-ray attachment

(EDX). The fractured surface observation was carried out after chemical etching with a

hydrofluoric acid solution for 3 minutes (HF - 2 vol %).

The samples in the natural condition and polished had their 3D view performed by

profilometry, as well as their roughness profiles. The equipment used was a confocal 3D

microscope (Nano Focus, model µsurf Explorer). The apparatus works based on non-contact

surface metrology with high-resolution sensors and linear encoders on X, Y and Z axes. The

100

investigated region was aways kept confined to an area of 1.5 mm x 1.5 mm located at the center

of each workpiece, as indicated by C letter in Fig. 48. A stencil was developed to ensure the

marking and measurements exactly in the same region of each part.


Fig. 49 exposes precise microstructural details collected from the standard composition and

those eco-friendly ones, i.e. containing CK wastes. In STD micrography, secondary mullite

(3Al2O3.2SiO2) was detected (needles shape) in the vitreous matrix, however, fewer concentrated

than in the others compositions. For the composition, 10%, the crystals of mullite secondary are

better distributed than in the others compositions and there are fewer quartz particles in the region

under evaluation. For C15 and C20, it is possible to verify that there are micro cracks on the quartz

particles as well as agglomerates of secondary mullite impregnated. The arrow in Fig. 49 indicates

the regions of concentration of secondary mullite.

Figure 49: SEM Micrographs of sintered samples from the compositions STD, 10%, 15% and

20%. (15,000x magnification).

101

The final surface quality of the STD composition can be evaluated by the glossiness and

roughness curves shown in Fig 50a and 50b, respectively. The maximum glossiness pattern

obtained for this composition reached the level of 59 GU, while the roughness pattern reached was

0.34 µm of average roughness Ra.

Figure 50: Glossiness and roughness curves for the STD composition.

The polishing performance of the eco-friendly compositions is evaluated through the gloss

and roughness curves shown in Figures 51 and 52, respectively. All three CK incorporations (C10,

C15 and C20) showed mean final glossiness values higher than that observed for the STD mixture.

According to Table 10, the compositions C10, C15 and C20 had the quartz content

decreased, reaching 0 wt.% for the compositions C15 and C20. Such substitution resulted in an

effective reduction in water absorption and apparent porosity levels in half, in the case of the

compositions C15 and C20 (still in Table 10), and a mullite enrichment in the microstructure, as

seen in Figure 49. After the polishing process, the eco-friendly compositions presented average

102

gloss values higher than the standard composition, with the composition C20 being the highest

glossiness average and showing the best topography of the polished surface (Figure 55b).

In this way, considering that all mixtures were polished under the same condition is

reasonable to state that the glossiness patterns for the eco-friendly compositions were probably

improved by the enrichment of mullite from the CK insertion.

103

Figure 51: Glossiness curves for the eco-friendly compositions.

104

The average roughness values for C10 and C20 compositions were below the STD, but

without large differences, the composition C15 had an average roughness higher than the others,

including the STD.

Important interactions occur through the polishing process (as ilustrated in Fig. 45) and it

surely influences considerably on the topography of the workpieces, providing the smoothness

Figure 52: Roughness curves for the eco-friendly compositions.

105

expected on the final surface. In spite of the final gloss and roughness, the 3D measurements were

also used as a tool to evaluate the final surface quality of the eco-friendly compositions. The

topographies are highlighted in Figures 53, 54 and 55. Additionally, the roughness profile at the

centre of the analyzed area was taken.

Figure 53: Topography of the composition C10: (a) Fired surface (natural condition) and (b)

polished surface.


surface.

As expected, a great difference in the topography between the samples under natural

conditions and polished was revelated. Comparatively, the composition C20 had presented the best

finish level among within the studied ones, as shown in Figure 55b, corroborating to the values of

glossiness reached in the respective gloss curve, shown in Figure 52.

106


surface.

Although polished, the surfaces still have shown remain pores, but the presence of such

pores is an intrinsic feature of the porcelain stoneware tiles polishing process. During the polishing

process some pores are completely removed, whereas others that were previoulsy closed become

open [18].

Additionally, Fig 56 shows the remaining microporosity in the samples of the eco-friendly

compositions, some pores, and agglomerates of pores was admitted as spherical and had their

diameters measured.

107

Figure 56: Optical image of the eco-friendly compositions.

Conclusion

Among the eco-compositions, C20 had the best final glossiness pattern showing an

improvement of 11GU in relation to the STD mixture and a positive variation of 4.5 GU and 3.6

GU in relation to C10 and C15, respectively.

For the polishing conditions employed here, the glossiness values were satisfactory and

showed that CK is interesting from the point of view of the polishing process. Its incorporation

besides saving natural raw materials can produce eco-friendly porcelain stoneware tiles with

excellent aesthetic values. Moreover, the final gloss pattern can certainly be improved by using

the ideal equipment for polishing, as well as optimizing the kinematic conditions of scratching,

abrasive sequence and ideal polishing time.

108

References

[1] J. E. Soares Filho, J. C. Aurich, F. J. P. Sousa, R. M. Nascimento, and C. A. Paskocimas,

“Estimation of the minimum material removal thickness during the polishing process of

ceramic tiles by laser triangulation,” Ceram. Int., no. July, pp. 0–1, 2017.

[2] H. P. A reiro, and D. A. Macedo, “Structural study of mullite based ceramics derived from

a mica-rich kaolin waste,” Ceram. Int., vol. 43, no. 4, pp. 3919–3922, Mar. 2017.

[3] R. R. Menezes, M. I. Brasileiro, W. P. Gonçalves, L. N. D. L. Santana, G. A. Neves, H. S.

Ferreira, and H. C. Ferreira, “Statistical design for recycling kaolin processing waste in the

manufacturing of mullite-based ceramics,” Mater. Res., vol. 12, no. 2, pp. 201–209, 2009.

[4] H. P. A. Alves, J. B. Silva, L. F. A. Campos, S. M. Torres, R. P. S. Dutra, and D. A. Macedo,

“Preparation of mullite based ceramics from clay–kaolin waste mixtures,” Ceram. Int., vol.

42, no. 16, pp. 19086–19090, Dec. 2016.


industrial wastes in ceramic manufacturing: State of art and glass case studies,” Ceram. Int.,

vol. 42, no. 12, pp. 13333–13338, 2016.

[6] P. Ter Teo, A. S. Anasyida, P. Basu, and M. S. Nurulakmal, “Recycling of Malaysia’s

electric arc furnace (EAF) slag waste into heavy-duty green ceramic tile,” Waste Manag.,

vol. 34, no. 12, pp. 2697–2708, 2014.

[7] B. C. A. Pinheiro and J. N. F. Holanda, “Reuse of solid petroleum waste in the manufacture

of porcelain stoneware tile,” J. Environ. Manage., vol. 118, pp. 205–210, 2013.

[8] S. Chitwaree, J. Tiansuwan, N. Thavarungkul, and L. Punsukumtana, “Energy saving in

sintering of porcelain stoneware tile manufacturing by using recycled glass and pottery

stone as substitute materials,” Case Stud. Therm. Eng., vol. 11, no. January, pp. 81–88,

2018.

[9] I. M. Hutchings, Y. Xu, E. Sánchez, M. J. Ibáñez, and M. F. Quereda, “Development of

surface finish during the polishing of porcelain ceramic tiles,” J. Mater. Sci., vol. 40, pp.

37–42, 2005.

[10] C. Y. Wang, T. C. Kuang, Z. Qin, and X. Wei, “How Abrasive Machining Affects Surface

Characteristics of Vitreous Cermamic Tile,” Am. Ceram. Soc., no. October, pp. 9201–9208,

2003.

[11] F. J. P. Sousa, L. L, and R. G, “Polishing,” in CIRP Encyclopedia of Production

Engineering, Springer-Verlag Berlin Heidelberg, 2014, pp. 957–962.

[12] V. Cantavella, E. Sánchez, M. J. Ibáñez, M. J. Orts, J. García-Ten, and A. Gozalbo,

109

“Grinding Work Simulation in Industrial Porcelain Tile Polishing,” Key Eng. Mater., vol.

264–268, no. 1, pp. 1467–1470, 2004.

[13] F. J. P. Sousa, J. C. Aurich, W. L. Weingaertner, and O. E. Alarcon, “Kinematics of a single

abrasive particle during the industrial polishing process of porcelain stoneware tiles,” J.

Eur. Ceram. Soc., vol. 27, no. 10, pp. 3183–3190, 2007.

[14] F. J. Pinheiro Sousa, J. Seewig, C. Chiamulera, O. E. Alarcon, and W. L. Weingärtner,

“Evolution of Wear on the Abrasive Tool during the Polishing of Porcelain Tile Using

Morphological Space,” Adv. Mater. Res., vol. 906, pp. 293–302, 2014.

[15] J. E. Soares Filho, A. de O. Feitosa, L. L. Santos, L. F. A. Campos, and R. P. S. Dutra,

“Study of the Effect of Feldspar Replacement from a Mixture of Glass / Syenite in

Technological Properties of Ceramic Coatings,” Mater. Sci. Forum, vol. 798–799, pp. 294–

299, 2014.

[16] A. de Oliveira Feitosa, J. E. S. Filho, L. L. dos Santos, R. R. Menezes, and R. P. S. Dutra,

Analysis of color of the ceramic coatings submitted to different processing conditions, vol.

798–799. 2014.

[17] I. M. Hutchings, K. Adachi, Y. Xu, E. Sánchez, M. J. Ibáñez, and M. F. Quereda, “Analysis

and laboratory simulation of an industrial polishing process for porcelain ceramic tiles,” J.

Eur. Ceram. Soc., vol. 25, no. 13, pp. 3151–3156, 2005.

[18] H. J. Alves, M. R. Freitas, F. G. Melchiades, and A. O. Boschi, “Dependence of surface

porosity on the polishing depth of porcelain stoneware tiles,” J. Eur. Ceram. Soc., vol. 31,

no. 5, pp. 665–671, 2011.

110

4 FINAL CONSIDERATIONS

This thesis has studied for the first time the influence of the microstructure on the polishing

process of porcelain stoneware tiles manufactured with industrial wastes. The information herein

generated was presented in four chapters, which provide the experimental subsidies, still missing

in the literature, for the development of eco-friendly porcelain stoneware tiles in the polished

version. This is a product of higher production cost among the other typologies of ceramic tiles,

with high benefit and aesthetic appeal.

The investigations in the manuscripts dealt with the characterization and evaluation of the

potentiality of insertion of different industrial wastes in ceramic matrix for high performance floor

tiles. An innovative pre-polishing study and the evaluation of the surface quality of the porcelain

stoneware tiles developed with residues, relating to the microstructural difference generated by the

insertion of the wastes.

In the chapter, “High-performance green ceramic tiles formulations with industrial

wastes: development and microstructural evaluation”, high-performance ceramic tiles were

designed based on the analysis of the technological and mechanical properties of the mixtures with

industrial wastes included in the composition. All the wastes considered in this investigation are

from the state of Rio Grande do Norte - Brazil and showed similar characteristics when compared

to main raw materials used in the manufacturing process of porcelain stoneware tile (feldspars,

clays, and quartz), being able to be used as substitute raw materials.

In the percentages listed in the manuscript, the wastes are perfectly capable to contributing

to the formation of the final microstructure as a source of mullite (fine kaolin with a higher

contribution than coarse kaolin), the liquid phase former (grog, rock powder and scheelite) and

inert material (coarse kaolin and grog showing higher contribution than the others). Moreover, the

environmental impact caused by the wrong disposal of such wastes is reduced.

Once the insertion capacity of the residues was verified and the development of ecological

porcelain tiles was successfully concluded, it was necessary to evaluate the different surfaces

generated before the polishing process. In this way, a topographic study of six different

compositions was carried out in the chapter "Estimation of the minimum material removal

thickness during the polishing process of ceramic tiles by laser triangulation".

An important benefit of this investigation is that the technique adopted in the pre-polishing

inspection (structured fringe projection profilometry) seems extremely useful in helping decision-

making in the production process. It provides precise information on the topography and the

minimum layer to be individually removed from the porcelain stoneware tile surfaces. The main

111

results have shown that the minimum removal layers were found to be between 1.08% and 2.37%

of the initial thickness in contrast to the values typically adopted in the ceramic industries

(approximately, 10% of the initial thickness is removed).

Subsequently, the polishing of the compositions was carried out under laboratory

conditions, with a sequence of 12 different abrasive grains, simulating a sequence of 36 industrial

polishing heads. The chapter “Polishing performance of eco-friendly ceramic tiles made with

wastes from bricks and roof tiles” has evaluated the influence of the insertion of waste from the

breakage of bricks and tiles on the polishing performance of three porcelain stoneware tiles

compositions with different proportions of waste (5%, 10%, 15%). According to the

microstructural evaluation carried out, the mullite regions were enriched with iron from the waste.

Probably, iron (in Fe+3 ions) is bounded in the mullite structure interstices.

The performance of the final surface was evaluated through the gloss and roughness curves,

as well as monitoring the morphological evolution of the surface in three different polishing steps.

The gloss levels were compared to the glossiness standard obtained after polishing of waste-free

composition (59 gloss units). Those values were compatible with commercial polished porcelain

stoneware tiles currently available in the market, up to 72.7 gloss units, as occurred for the

composition C10.

In the chapter “Evaluation of surface quality after polishing of eco-friendly porcelain

stoneware tiles made with kaolin ore waste”, the waste from Kaolin beneficiation process served

as a substitute of quartz in the formulations in three different amounts (10%, 15%, and 20%). After

polishing of the compositions (under the same conditions of the previous chapter) the quality of

the polished surfaces were evaluated by analysis of the glossiness and roughness curves, as well

as by the 3D topography.

Those substitutions resulted in an effective reduction in water absorption and apparent

porosity levels. This was verified qualitatively by scanning electron microscopy, and a mullite

enrichment in the microstructure was detected. After polishing, the eco-friendly compositions

presented average gloss values higher than the standard composition (59 gloss units). The highest

glossiness reached was 70 gloss units for the composition C20, which represented an increase of

18.6% in the final surface glossiness in comparison with the standard composition.

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5 CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK

5.1 CONCLUSIONS

In this thesis, porcelain stoneware tiles with industrial wastes have been developed, and

their microstructural and surface characterized after polishing. The used wastes were introduced

in the compositions, in different proportions, as alternative raw materials combined with the

traditional ones aiming to produce eco-friendly porcelain stoneware tiles. In this way, the influence

of the microstructure on the polishing performance was investigated. Based on the experimental

results, the following conclusions can be drawn:

The use of laser profilometry emerged as a suitable pre-polishing surface evaluation tool.

It is possible to provide industries with an estimate of the minimum thickness removal

during the polishing process. The technique can be of great value in the most costly stage

of the production process, avoiding the misuse of polishing conditions in real polishing

lines, for reducing the consumption of abrasive tooling, energy, and lubricating fluid.

The waste from the breakage of bricks and roof tiles has provided an extra source of iron,

enriching the mullite structure interstices. The introduction of such waste was able to reach

good final surface gloss levels, up to 72.7 gloss units, as seen for the composition with 10

wt.% of waste.

Among the eco-compositions with the waste from the kaolin beneficiation process, the

mixture with 20 wt.% of waste had the best final glossiness pattern showing an

improvement of 11 GU in relation to the STD mixture and a positive variation of 4.5 GU

and 3.6 GU in relation to the blends with 10 wt.% and 15 wt.% of waste, respectively.

For the polishing conditions employed here, the glossiness values were satisfactory and

showed that the wastes from the breakage of bricks a nd roof tiles and from the kaolin

beneficiation process are interesting from the point of view of the polishing process in

different percentages of incorporation.

All the incorporations herein tested, besides saving natural raw materials can produce eco-

friendly porcelain stoneware tiles with excellent aesthetic values.

The final gloss pattern can certainly be improved by using the ideal equipment for

polishing, as well as optimizing the kinematic conditions of scratching and abrasive

sequence.

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The experimental results point out that innovative eco-friendly ceramic tiles reaching

excellent aesthetic performance can be produced by the incorporation of industrial wastes from

the state of Rio Grande do Norte, Brazil. This alternative not only saves original raw materials but

also contribute to a circular economy, adding value to industrial wastes currently being used simply

as landfill in civil construction or otherwise wrongly disposed of in the environment.

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5.2 SUGGESTIONS FOR IMPROVEMENTS AND FUTURE INVESTIGATIONS

As already mentioned, this thesis stands out for of innovative eco-friendly ceramics tiles.

Furthermore, the experimental results have shown the capability of reusing the industrial wastes

in the manufacturing of products technically more advanced. In this way, some suggestions for

improvements and future investigations are listed:

- In order to obtain higher values of gloss using the same wastes herein tested, further additions

will be still possible, and a limit may be reached, above which the polishing is no longer of interest

from the aesthetic point of view.

- Aiming to increase the range of eco-friendly ceramic tiles, new industrial wastes may be tested,

resulting in different microstructures to be polished in order to achieve the gloss pattern for each

type of waste.

- Further investigations considering the glossiness and roughness equations as proposed by

Hutctings should be carried out with the support of the specific machinery for polishing porcelain

stoneware tiles, including all industrial movements available, as well as optimizing the kinematic

conditions of scratching, abrasive sequence and polishing time.

- Considering the industrial production line, a 3D mapping could be performed on commercial

porcelain stoneware tiles from different regions of the firing furnace, obtaining an empirical

correlation between the temperature gradients in the furnaces and the surface characteristics, thus

facilitating the planning of the subsequent polishing.

Ph.D. THESIS - UFRN

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